MrDocs reference documentation

[alandefreitas]

#665 caught a bug when trying to document MrDocs with MrDocs. There's an old custom target in CMakeLists.txt that attempts to do that, but we don't use this output anywhere else.

We have to discuss what to do about it.

• Since we're not using it anywhere, we could remove this logic everywhere. However, it's weird that we don't check if MrDocs can document itself and that other people will tell us about bugs in it.
• Or we could start using it somewhere, like the documentation or as a demo.
• Or we could not use it anywhere but still check in CI if it works. We can run that with the demos but not upload it.