`pkgdown` implementation for `datr`

It seems like a good idea to use pkgdown to make a cool landing page for the datr package. This will then become the page to reference to.

I am unsure whether the packages contain sufficient information already to do this, because I haven’t looked into this further. Contributions welcome if anyone is interested in helping out :slight_smile:

Also added this one immediately after the retractcheck package one, so it can go off the to do list :slight_smile:

pkgdown::build_site() really is super easy :confetti_ball: Something to add to the developer operations for developing R packages :slight_smile: