Use PyData Sphinx Theme docs version switcher

[matthewfeickert]

From the documentation for the theme (https://pydata-sphinx-theme.readthedocs.io/en/latest/user_guide/readthedocs.html#version-switcher) the version switcher from readthedocs should still work after this is done (but won't show up before the PR is merged). If you want to use the version switcher of the PyData Sphinx Theme instead, it would require having urls for all of the previous versions of the library and one json file mapping urls to versions. This would also enable using the Version warning banner from the PyData theme itself: https://pydata-sphinx-theme.readthedocs.io/en/latest/user_guide/announcements.html#version-warning-banners

From https://pyhf.readthedocs.io/en/latest/ seems like we'll need the to use the PyData Sphinx Theme version switcher.

Originally posted by @matthewfeickert in #2513 (comment)