I don't think this is feature already included in pkgdown. Also, non-exported functions are most often not documented, so no help page. The reference page would just be a list of non exported function ?
If so, you could with current version built an article showing all the non exported functions in your package
Here is an example on how to get all names of non exported functions
# load the package you want to extract function from
package_to_analyze <- "pkgdown"
library(package_to_analyze, character.only = T)
# get only exported function using the loaded package env name
exported_funs <- unclass(lsf.str(paste0("package:", package_to_analyze), all.names = TRUE))
# get all functions inside the package using asNamespace
all_funs <- unclass(lsf.str(envir = asNamespace(package_to_analyze), all.names = T))
# get only non exported funs
setdiff(all_funs, exported_funs)
#> [1] "[.tag"
#> [2] "a"
#> [3] "add_github_links"
#> [4] "add_slug"
#> [5] "all_sign"
#> [6] "article_index"
#> [7] "article_index_local"
#> [8] "article_index_remote"
#> [9] "as_data"
#> [10] "as_data.NULL"
#> [11] "as_data.tag_arguments"
#> [12] "as_data.tag_description"
#> [13] "as_data.tag_details"
#> [14] "as_data.tag_examples"
#> [15] "as_data.tag_format"
#> [16] "as_data.tag_item"
#> [17] "as_data.tag_note"
#> [18] "as_data.tag_references"
#> [19] "as_data.tag_section"
#> [20] "as_data.tag_seealso"
#> [21] "as_data.tag_source"
#> [22] "as_data.tag_usage"
#> [23] "as_data.tag_value"
#> [24] "as_html"
#> [25] "as_html.character"
#> [26] "as_html.COMMENT"
#> [27] "as_html.LIST"
#> [28] "as_html.NULL"
#> [29] "as_html.RCODE"
#> [30] "as_html.Rd"
#> [31] "as_html.tag"
#> [32] "as_html.tag_acronym"
#> [33] "as_html.tag_bold"
#> [34] "as_html.tag_cite"
#> [35] "as_html.tag_code"
#> [36] "as_html.tag_command"
#> [37] "as_html.tag_concept"
#> [38] "as_html.tag_cr"
#> [39] "as_html.tag_deqn"
#> [40] "as_html.tag_describe"
#> [41] "as_html.tag_dfn"
#> [42] "as_html.tag_dontrun"
#> [43] "as_html.tag_dontshow"
#> [44] "as_html.tag_donttest"
#> [45] "as_html.tag_dots"
#> [46] "as_html.tag_dQuote"
#> [47] "as_html.tag_email"
#> [48] "as_html.tag_emph"
#> [49] "as_html.tag_enc"
#> [50] "as_html.tag_enumerate"
#> [51] "as_html.tag_env"
#> [52] "as_html.tag_eqn"
#> [53] "as_html.tag_figure"
#> [54] "as_html.tag_file"
#> [55] "as_html.tag_href"
#> [56] "as_html.tag_if"
#> [57] "as_html.tag_ifelse"
#> [58] "as_html.tag_item"
#> [59] "as_html.tag_itemize"
#> [60] "as_html.tag_kbd"
#> [61] "as_html.tag_ldots"
#> [62] "as_html.tag_link"
#> [63] "as_html.tag_linkS4class"
#> [64] "as_html.tag_method"
#> [65] "as_html.tag_newcommand"
#> [66] "as_html.tag_option"
#> [67] "as_html.tag_out"
#> [68] "as_html.tag_pkg"
#> [69] "as_html.tag_preformatted"
#> [70] "as_html.tag_R"
#> [71] "as_html.tag_renewcommand"
#> [72] "as_html.tag_S3method"
#> [73] "as_html.tag_S4method"
#> [74] "as_html.tag_samp"
#> [75] "as_html.tag_Sexpr"
#> [76] "as_html.tag_sQuote"
#> [77] "as_html.tag_strong"
#> [78] "as_html.tag_subsection"
#> [79] "as_html.tag_tab"
#> [80] "as_html.tag_tabular"
#> [81] "as_html.tag_testonly"
#> [82] "as_html.tag_url"
#> [83] "as_html.tag_var"
#> [84] "as_html.tag_verb"
#> [85] "as_html.TEXT"
#> [86] "as_html.USERMACRO"
#> [87] "as_html.VERB"
#> [88] "author_desc"
#> [89] "author_list"
#> [90] "author_name"
#> [91] "autolink_license"
#> [92] "badges_extract"
#> [93] "badges_extract_text"
#> [94] "bib_paths"
#> [95] "bst_paths"
#> [96] "build_articles_index"
#> [97] "build_authors"
#> [98] "build_citation_authors"
#> [99] "build_cname"
#> [100] "build_docsearch_json"
#> [101] "build_home_index"
#> [102] "build_home_license"
#> [103] "build_home_md"
#> [104] "build_logo"
#> [105] "build_news_multi"
#> [106] "build_news_single"
#> [107] "build_reference_topic"
#> [108] "build_rmarkdown_format"
#> [109] "build_site_external"
#> [110] "build_site_local"
#> [111] "build_site_meta"
#> [112] "build_sitemap"
#> [113] "cat_line"
#> [114] "check_made_by"
#> [115] "cname_url"
#> [116] "construct_commit_message"
#> [117] "context_get"
#> [118] "context_set"
#> [119] "context_set_scoped"
#> [120] "copy_asset_dir"
#> [121] "copy_assets"
#> [122] "copy_favicons"
#> [123] "create_meta"
#> [124] "data_articles_index"
#> [125] "data_articles_index_section"
#> [126] "data_author_info"
#> [127] "data_authors"
#> [128] "data_citations"
#> [129] "data_home"
#> [130] "data_home_sidebar"
#> [131] "data_home_sidebar_authors"
#> [132] "data_home_sidebar_citation"
#> [133] "data_home_sidebar_license"
#> [134] "data_home_sidebar_links"
#> [135] "data_navbar"
#> [136] "data_news"
#> [137] "data_open_graph"
#> [138] "data_reference_index"
#> [139] "data_reference_index_section"
#> [140] "data_reference_topic"
#> [141] "default_articles_index"
#> [142] "default_reference_index"
#> [143] "defer"
#> [144] "deploy_local"
#> [145] "dev_mode"
#> [146] "devtools_loaded"
#> [147] "devtools_meta"
#> [148] "digest_plot"
#> [149] "dir_copy_to"
#> [150] "dir_depth"
#> [151] "dont_index"
#> [152] "drop_leading_newline"
#> [153] "dst_path"
#> [154] "escape_html"
#> [155] "extract_package_attach"
#> [156] "extract_package_attach_"
#> [157] "extract_source"
#> [158] "extract_tag"
#> [159] "extract_title"
#> [160] "fetch_yaml"
#> [161] "fig_name"
#> [162] "fig_opts_chunk"
#> [163] "fig_save"
#> [164] "fig_save_default"
#> [165] "file_copy_to"
#> [166] "file_digest"
#> [167] "file_equal"
#> [168] "find_article"
#> [169] "find_icon"
#> [170] "find_icons"
#> [171] "find_logo"
#> [172] "find_qualifier"
#> [173] "find_rdname"
#> [174] "find_rdname_attached"
#> [175] "find_rdname_local"
#> [176] "find_reexport_source"
#> [177] "find_reexport_source_from_imports"
#> [178] "find_template"
#> [179] "find_tutorials"
#> [180] "flatten_para"
#> [181] "flatten_text"
#> [182] "format_author_name"
#> [183] "format_example_chunk"
#> [184] "fun_info"
#> [185] "fun_name"
#> [186] "git"
#> [187] "github_clone"
#> [188] "github_push"
#> [189] "github_source"
#> [190] "github_source_links"
#> [191] "github_url_rx"
#> [192] "has_citation"
#> [193] "has_favicons"
#> [194] "has_internet"
#> [195] "has_news"
#> [196] "highlight_text"
#> [197] "href_article"
#> [198] "href_expr"
#> [199] "href_expr_"
#> [200] "href_string"
#> [201] "href_tokens"
#> [202] "href_topic"
#> [203] "href_topic_local"
#> [204] "href_topic_remote"
#> [205] "invert_index"
#> [206] "is_dev"
#> [207] "is_infix"
#> [208] "is_internal"
#> [209] "is_low_change"
#> [210] "is_newline"
#> [211] "is_non_pkgdown_site"
#> [212] "is_pkgdown"
#> [213] "is_prefix"
#> [214] "is_syntactic"
#> [215] "is_testing"
#> [216] "label_lines"
#> [217] "label_output"
#> [218] "licenses_db"
#> [219] "link_url"
#> [220] "linkify"
#> [221] "made_by_pkgdown"
#> [222] "make_slug"
#> [223] "markdown"
#> [224] "markdown_text"
#> [225] "match_env"
#> [226] "match_eval"
#> [227] "match_fun"
#> [228] "menu"
#> [229] "menu_icon"
#> [230] "menu_link"
#> [231] "menu_links"
#> [232] "menu_spacer"
#> [233] "menu_text"
#> [234] "merge_low_plot"
#> [235] "meta_development"
#> [236] "meta_figures"
#> [237] "method_usage"
#> [238] "navbar_components"
#> [239] "navbar_news"
#> [240] "navbar_structure"
#> [241] "orcid_link"
#> [242] "out_of_date"
#> [243] "package_rd"
#> [244] "package_topics"
#> [245] "package_tutorials"
#> [246] "package_vignettes"
#> [247] "parse_descriptions"
#> [248] "parse_github_link"
#> [249] "parse_items"
#> [250] "parse_opts"
#> [251] "parse_section"
#> [252] "parse_usage"
#> [253] "path_abs"
#> [254] "path_first_existing"
#> [255] "path_package_pkgdown"
#> [256] "path_pkgdown"
#> [257] "pkg_authors"
#> [258] "pkg_github_url"
#> [259] "pkg_timeline"
#> [260] "pkgdown_detective"
#> [261] "pkgdown_renderer"
#> [262] "prepend_class"
#> [263] "print.print_yaml"
#> [264] "print.Rd"
#> [265] "print.tag"
#> [266] "print_yaml"
#> [267] "rd_file"
#> [268] "rd_text"
#> [269] "read_citation"
#> [270] "read_desc"
#> [271] "read_file"
#> [272] "read_lines"
#> [273] "read_meta"
#> [274] "readLines"
#> [275] "register_attached_packages"
#> [276] "remote_metadata"
#> [277] "remote_package_article_url"
#> [278] "remote_package_reference_url"
#> [279] "remote_urls"
#> [280] "remove_name"
#> [281] "render_index"
#> [282] "render_md"
#> [283] "render_navbar_links"
#> [284] "render_rmarkdown"
#> [285] "render_template"
#> [286] "replay_html"
#> [287] "replay_html.character"
#> [288] "replay_html.error"
#> [289] "replay_html.list"
#> [290] "replay_html.message"
#> [291] "replay_html.NULL"
#> [292] "replay_html.recordedplot"
#> [293] "replay_html.source"
#> [294] "replay_html.value"
#> [295] "replay_html.warning"
#> [296] "repo_link"
#> [297] "rmarkdown_template"
#> [298] "rstudio_save_all"
#> [299] "rule"
#> [300] "same_contents"
#> [301] "scoped_file_context"
#> [302] "scoped_in_pkgdown"
#> [303] "scoped_package_context"
#> [304] "section_init"
#> [305] "select_topics"
#> [306] "select_vignettes"
#> [307] "set_class"
#> [308] "set_classes"
#> [309] "set_contains"
#> [310] "set_pkgdown_env"
#> [311] "short_name"
#> [312] "sidebar_section"
#> [313] "skip_if_no_pandoc"
#> [314] "split_at_linebreaks"
#> [315] "src_path"
#> [316] "stop_bad_tag"
#> [317] "str_person"
#> [318] "str_trim"
#> [319] "strip_html_tags"
#> [320] "tag"
#> [321] "tag_insert"
#> [322] "tag_wrapper"
#> [323] "template_path"
#> [324] "tex_paths"
#> [325] "topic_funs"
#> [326] "topic_index"
#> [327] "topic_index_installed"
#> [328] "topic_index_local"
#> [329] "topic_must"
#> [330] "trim_ws_nodes"
#> [331] "tutorial_info"
#> [332] "tweak_anchors"
#> [333] "tweak_code"
#> [334] "tweak_code_nodeset"
#> [335] "tweak_homepage_html"
#> [336] "tweak_md_links"
#> [337] "tweak_news_heading"
#> [338] "tweak_pre_node"
#> [339] "tweak_rmarkdown_html"
#> [340] "tweak_tables"
#> [341] "up_path"
#> [342] "update_html"
#> [343] "url_node"
#> [344] "usage_code"
#> [345] "usage_code.COMMENT"
#> [346] "usage_code.NULL"
#> [347] "usage_code.RCODE"
#> [348] "usage_code.Rd"
#> [349] "usage_code.tag"
#> [350] "usage_code.tag_dots"
#> [351] "usage_code.tag_method"
#> [352] "usage_code.tag_S3method"
#> [353] "usage_code.tag_S4method"
#> [354] "usage_code.tag_usage"
#> [355] "usage_code.TEXT"
#> [356] "usage_code.VERB"
#> [357] "usage_type"
#> [358] "with_device"
#> [359] "with_dir"
#> [360] "write_if_different"
#> [361] "write_lines"
#> [362] "write_yaml"
#> [363] "writeLines"
#> [364] "yaml_list"
detach(paste0("package:", package_to_analyze), character.only = TRUE)
Created on 2019-04-20 by the reprex package (v0.2.1.9000)
You can then continue filtering to not keep method for example, and work on how to build a web page to present those results (using rmarkdown, html, ...
It is just to give ideas.