Switch to plone-sphinx-theme

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](https://github.com/plone/plone-sphinx-theme) 2.0 will be released soon and is based on [PyData Theme](https://pydata-sphinx-theme.readthedocs.io/en/stable/). 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.

Start by finding the Sphinx and Read the Docs theme configuration, then review issue #107 and the plone-sphinx-theme 2.0 release status. Update the configured theme after 2.0 is available and verify that the documentation builds and provides the intended accessibility and scroll-spy improvements.

Written by the indexing model from the issue text.