Replace GitHub Pages documentation site with index.html that redirects to ReadTheDocs

[mfisher87]

Important

Note that this PR targets the new-gh-pages branch. This is an orphan branch created with git checkout --orphan new-gh-pages. Then, an empty commit was created with git commit --allow-empty. Then, my feature branch was taken from this new HEAD. This simulates the nature of the gh-pages branch in a way that is reviewable. Once this PR is approved and merged, I'll replace the gh-pages branch with the contents of the new-gh-pages branch and squash to one commit. From then on, changes to gh-pages can be done with PRs.

This PR enables redirect to ReadTheDocs and decommissioning of the docs GitHub Action (see #277, #362).

Should there be a README on this branch? The index.html file is pretty self-explanatory.