Use ReadTheDocs

Currently we're using Github Pages for hosting and Netlify for PR previews. We keep having issues due to Netlify's Python environment, and it's lack of support for Conda making our packages fall out of sync.

ReadTheDocs now supports Conda/Mamba, so we could simplify our environment, and have the same service manage the build and PR previews.

I remember we had some discussion about RTD before, and I can't remember exactly why we didn't use them and instead built our own versioning scheme. Was it due to the advertising? We can opt it out of advertising by someone getting a membership, which I just tried for Xpublish.

To get our existing old sites, we would probably want to tag the desired commits, then use post_build customization to pull those in. The other place to customize might be to use single version mode.

xref: #237 @emiliom @ocefpaf

Start by reviewing the current GitHub Pages and Netlify configuration, then read the prior discussion in #237 and the linked ReadTheDocs documentation for Conda, post_build, and single-version mode. Done would require an agreed migration plan covering builds, PR previews, and existing old sites; no implementation file or test is named.