Significant improvements:
* Allow easy isolated local builds via a dedicated Tox environment
* Configure Sphinx to use consistent packages and theme across local, CI,
and ReadTheDocs builds; local builds now look the same as the published
RTD pages
* Explicitly add a ReadTheDocs configuration as code, per their documented
best practices
* Remove lots of dead code and simplify the Sphinx configuration file
* Build docs in a dedicated CI step and enable stricter warning checking
