One of the websites that consistently comes up in R-related search results is rdrr.io. It hosts the man pages and vignettes that you can easily view in the browser. What I recently noticed is that the vignettes are only available for CRAN packages, but not the GitHub ones, even if it is the same package. For example, https://rdrr.io/cran/tidyr/ vs https://rdrr.io/github/tidyverse/tidyr/. Is there some service that generates the vignettes? One potential option is rdocumentation.org, but it does not appear to be actively maintained (for example, dplyr's latest available version is almost a year out of date).
rseek searchs. E.g.