Consider moving to the PyData Sphinx Theme

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.