Replace recommonmark in favor of MyST-Parser

[probonopd]

Sooner or later we may have to go through the hassle of replacing recommonmark in favor of MyST-Parser.
readthedocs/recommonmark#221

Since I dread this type of maintenance chores of what previously was a working system, I'd happily accept a pull request.

[Sporiff]

I recently went through this for the Funkwhale documentation as we migrated from rST to MyST. I would also recommend looking into using Sphinx design to get some really powerful features like tabbed code blocks etc.

I'm happy to take a look at this at some point.