[Docs] Update pydata-sphinx-theme to 0.16.x

Update our dev docs build to use the latest pydata-sphinx-theme (with 0.15.2 out, the main bug in 0.15.0 should be fixed, so I think we can at least start using it for the dev docs: https://github.com/pydata/pydata-sphinx-theme/releases/tag/v0.15.2)