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

まず現在の GitHub Pages と Netlify の設定を確認し、次に #237 の以前の議論と、Conda、post_build、single-version mode に関するリンク先の ReadTheDocs ドキュメントを読んでください。完了条件は、ビルド、PR プレビュー、既存の古いサイトを対象とする合意済みの移行計画が策定されていることです。実装ファイルやテストは指定されていません。