MyST markdown support

[mgielda]

This should be easy via docutils.

[mgielda]

Seems like https://github.com/executablebooks/markdown-it-py is now the recommended CommonMark md parser, which we should use instead of our own trivial implementation (this could also enable us to use the code block info string to pass params, instead of the comments hack -- syntax TBD). Also, it's from the MyST folks, so should enable us to do MyST pretty seamlessly.