Switch to building docs from GitHub Action

[stefmolin]

Currently, the docs are only built for the latest version of the code (HEAD), and previous versions are preserved automatically by virtue of publishing via the gh-pages branch. I'd like to switch to building the docs via a GitHub Action (removing the gh-pages branch), while still providing documentation for the last 2 versions (all are tagged).

The action needs to do the following:

1. Build the docs for HEAD (see the current action).
2. Find the two previous tagged versions and build the docs for each of those.
3. Publish everything.