Add vignettes (back?) to pkgdown website

[Aariq]

The vignettes/articles aren't in the navbar on our site. This may be due to some changes in how ropensci builds pkgdown websites. Looks like we need to explicitly add them. Here's an example: https://github.com/ropensci/quadkeyr/blob/main/_pkgdown.yml

[Aariq]

Hmm.. taht might not be the issue after all. hydroscoper has an "Articles" navbar item but it is not defined in their pkgdown.yml: https://github.com/ropensci/hydroscoper/blob/master/_pkgdown.yml

[Aariq]

Whoops, the issue was just that the vignette named .Rmd gets used for the "Getting Started" tab.