Add MyST

[maehr]

Add https://myst-parser.readthedocs.io/en/latest/ to converters.

@ashwinvis Could also be used to replace RSt in our list altogether ;)

[ashwinvis]

I have a bittersweet relation with rST. It is more standardised, and extendable than markdown, but it can throw a lot of errors too.

Also related to #19, which is the frontend for MyST. Have you tried it?

[maehr]

I am currently converting the docs of an older repo to md and expose it via sphinx. Seems to work pretty well.
https://myst-parser.readthedocs.io/en/latest/api/sphinx_parser.html