Add version dropdown to sphinx website

[JoeZiminski]

We should add a version dropdown to the sphinx website such that documentation for a specific version can be easily accessed:
https://pydata-sphinx-theme.readthedocs.io/en/stable/user_guide/version-dropdown.html

[JoeZiminski]

See https://blog.esciencecenter.nl/versioned-documentation-using-only-github-actions-and-github-pages-1825296e31aa

[niksirbi]

Aw this looks promising, it's something I wanted to solve for ages but never found the time to look into.