Add rworkflows

[bschilder]

A shameless plug for my new R-package dedicated GitHub Action, rworkflows. It basically streamlines all the checking, website creation, and even adds GH-native containerisation (without any manual steps).

The way it creates documentation website is by running pkgdown::build_site() each time the workflow is triggered (i.e. whenver you make a push), and pushing it to a new branch called "gh-pages". This means you don't need to store the "docs" folder in the main branch anymore (or run manual updates).

I also added some things peripherally related to rworkflows:

• Added some lines to the .Rbuildignore to avoid common pitfalls with GHA.
• Added some badges to display the current status of the rworkflows GHA, and show off your impressive code coverage (currently at 93%+!).

Let me know if you have any questions or requests for further tweaks.

[bschilder]

All checks are passing now, except for the ubuntu-latest, but that's a GH permissions thing. Should resolve itself if the PR is merged.