Now that recommonmark is deprecated https://github.com/readthedocs/recommonmark/issues/221,
myst-parser seems to be the generally accepted drop in replacement.
This swaps recommonmark out of the sphinx config and upgrades any
markdown document that needs adjustment due to myst-parser's stricter
In particular, these things needed addressed as myst-parser complained
- Broken references were replaced
- Invalid lexers in code blocks were replaced or removed (It didn't look like they were being syntax highlighted anyway)
- Non-contiguous headers (# -> #) were made contiguous (# -> )