We used recommonmark so that we can use markdown in sphinx but it is
buggy and now so switch to better supported MyST-parser.
* Fixed incorrect heading warnings in markdown.
* Added sphinx toctree to markdown using directive as required by MyST.
* Upgraded Sphinx to 4.3
Ref: https://myst-parser.readthedocs.io
- Use sphinxcontrib.spelling with custom wordlist.
- Skip the checking of the modules documents as they raise
false-positives.
- Add a setup.py spellcheck_docs command.
- Fix spelling and other issues.
- Add a doc favicon.