Add module documentation

Current version is less exciting: it just adds a \page for each module that needs to be manually filled in. Autogenerating is useful but if we want decent documentation, using actual documentation is probably better.

This is just a PoC to see if there's interest for this.

This patch adds a second function to the modules pipewire__module_describe which is (usually) a one-liner for the existing modules. It allows a helper tool to extract the module description and usage and push it into the doxygen documentation, producing a \page that lists all the known modules including author, version and usage information.