Update rake ghpages task

[nevans]

If the GitHub pages source is set to the "gh-pages" branch and the "/doc" folder, then this should get everything back up-to-date.

The older version should probably work, too. But I never like running rm -rf * if I can avoid it! I think the other changes are minor improvements, too.

It should be possible to run this rake task from a GitHub action.

---

I ran the following:

bundle exec rake ghpages RDOC_DOCUMENTED_BRANCH=v6.3.2
git push

And the output can be seen here: https://nevans.github.io/rdoc

FYI: this is my GitHub Pages configuration: