Switch to plone-sphinx-theme

[stevepiercy]

The Read the Docs Sphinx theme was a great step forward in usability, but lacks many of the nice features of modern Sphinx themes, such as Furo and PyData Theme, specifically scroll-spy to help readers see where they are within a page of documentation.

plone-sphinx-theme 2.0 will be released soon and is based on PyData Theme. When it's released, I'd suggest switching to it for better accessibility and usability. Its current released version includes a second theme, whereas 2.0 removes it, which is why I'd prefer to delay this implementation.

See also #107.