[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

Lies die in der Issue verlinkte Read the Docs-Dokumentation zu Versionen und untersuche anschließend die vorhandene Read the Docs-Konfiguration des Repositorys. Aktiviere auswählbare ältere Dokumentationsversionen und konfiguriere sie so, dass sie statt Branches Tags verwenden; die Aufgabe ist erledigt, wenn ältere mypy-Dokumentation auf mypy.readthedocs.io verfügbar ist, ohne auf veraltete Branches angewiesen zu sein.