sphinx: Allow markdown parsing

Not sure what the plans are for moderncmakedomain so I am also openning a MR here.

Basically this MR fixes the cmake-module so that it can properly parse any backend.

There are still more steps needed to make moderncmakedomain more usable to end-user, but in a nutshell, a good reference is matlab's implementation.

Pinging @henryiii for discussion on this, although maybe this should be in an issue.