Upgrade docs to Sphinx 3+ and add doc build test (#1613)

* Upgrade docs to Sphinx 3+
* Fix all the warnings...

- Fixed bad docstrings
- Fixed bad fenced code blocks in documentation
- Blocklisted some sections from being generated from the README
- Added missing documentation to index.rst
- Fixed an invalid autofunction directive in reference/reference_functions.rst
- Pin another documentation dependency

* Add documentation build test