[docs feature] keep more readthedocs versions around

Currently on https://mypy.readthedocs.io/ you can switch between stable and latest versions of the documentation, which is very convenient. However, what if you want to look at an older version of the documentation, such as if your installed mypy version is older or perhaps you want to track down when something changed in mypy history? Not so convenient now, because you can't go look at old versions of the documentation. However, this seems to be a feature that one can enable in readthedocs, called (in general) "versions" https://docs.readthedocs.com/platform/latest/versions.html and it seems to be free for community users as well https://about.readthedocs.com/pricing/#/community ("Documentation versioning", here, not to be confused with "build concurrency"), so, that seems like a useful thing to do.

On another note, the old versions should be configured to use tags not branches, so that we can get rid of stale branches à la https://github.com/python/mypy/issues/19795

Issue にリンクされている Read the Docs の versions ドキュメントを読み、その後、リポジトリに既存の Read the Docs 設定を確認します。選択可能な古いドキュメントバージョンを有効にし、ブランチではなくタグを使用するよう設定します。古い mypy のドキュメントが、古いブランチに依存せず mypy.readthedocs.io で利用できれば完了です。