Add a version selector widget

[agoose77]

At SciPy 2024, there were users who wanted a version selector to move between deployed versions of a static site (such as @mwcraig). We could adopt the pydata-sphinx-theme approach to add support for this feature as a simple <a href> dropdown.

This also might relate to readthedocs/readthedocs.org#11474 (comment), which amongst other things drops the flyout HTML injection and exposes version information through a custom event

To satisfy both cases, we might want to be smart and bundle RTD + explicit versioning in a single theme plugin.

[mwcraig]

Thanks for your help getting this going for me at SciPy 😀

[humitos]

I implemented the version/language selectors for the PyData Sphinx theme as an example. You can take a look at it in this comment: pydata/pydata-sphinx-theme#705 (comment)