readthedocs version selector

Maybe interesting for moin also?

https://moin-20.readthedocs.io/en/latest/ - see there at the lower right.

https://borgbackup.readthedocs.io/en/latest/ - there it sits at top left above the search field and also the page is a bit more wide.

Changeset: https://github.com/borgbackup/borg/pull/9428