Officially, only the manpage role is supported at the moment.

Unlike in rST, the syntax uses braces instead of colons:

    {manpage}`nix.conf(5)`

Switch to CommonMark with our extensions.

doc: Clean up generated media directory

Syntax is taken from MyST:

https://myst-parser.readthedocs.io/en/latest/using/syntax.html#targets-and-cross-referencing

We were using Pandoc’s Markdown parser. This is non-standard and might cause issues when we finally switch to some other tool.

Since RFC 0072 agreed on CommonMark, let’s switch to that.

We will use some extensions to make the limited syntax bearable but these are official so they should be more portable.

pandoc uses ./media to extract media files - add it to the list
of directories that need to be removed by `make clean`.