Did you put a section articles in your _pkgdown.yml ?
You should be able to tweak the articles section this way :
articles:
- title: All vignettes
contents:
- Introduction_to_tidyLPA
- comparing-mclust-and-mplus-output
See documentation for more details and try on your site.
You can also follow some example as the dplyr website where articles are organized in the navbar.