Consider moving to the PyData Sphinx Theme

[mdickinson]

We should consider moving ETS projects from the enthought-sphinx-theme to the PyData Sphinx Theme: https://github.com/pydata/pydata-sphinx-theme

My main motivation here is to move away from the Enthought Sphinx theme, to save us from the (not inconsiderable) pain of keeping that theme up to date and usable from a variety of different Sphinx versions. Sphinx theme maintenance doesn't seem like the best use of the limited resources available for ETS development.

I don't care so much what theme we move to, except that it should be something that's well maintained and has a reasonable likelihood of staying up-to-date and working with new Sphinx releases. The themes shipped with Sphinx itself are going to be the safest candidates in that respect, but any widely-used 3rd party theme should be fairly safe, too. The PyData Sphinx Theme and the Read the Docs theme are the two most obvious candidates there.