Empty github pages: Improve documentation on use_pkgdown_github_pages()

[valentingar]

Problem: Running use_pkgdown_github_pages() in a different branch than root leads to correct github workflow which however create an empty site and leads to a 404 error message when accessing the site.

Solution: The documentation in use_pkgdown_github_pages() should inform the user that the changes need to be pushed to the main branch of the repository.

How to reproduce Run use_pkgdown_github_pages() in a branch other than the root branch for a package where pkdown is not yet configured.

Usethis is great because it enables users without deep understanding of the underlying processes - like me - to do really useful things. In order to test out pkdown deployment, I first wanted to do so in a different branch to prevent any hiccups. Of course this lead to a missing _pkgdown.yml in the main branch so that the github workflow created an empty site. Because the process did not produce an error message and the github workflow ran successful, it wasnt immediately obvious what the problem was to me. Pushing the changes to the main branch fixed the problem.

Thanks a lot for your great work!!