Automatically update pkgdown site by GitHub Actions

[billdenney]

Related to #5

The current pkgdown site does not appear to be as functional as it was when first generated.

I think it would be beneficial to start using GitHub actions for pkgdown building. The simple way to do this is to run the following command:

usethis::use_pkgdown_github_pages()

@jackwasey, I believe that you will need to run the command since it makes some more GitHub changes to the repo. It will have a side-benefit that the pkgdown site can be removed from the main repo files after the changes are made.