The Doxygen "Modules" page is not very illuminative, as different parts
of the API are mixed together and not all parts are included.
Try to address this:
Put all parts of the public API to some Doxygen group, usually one group
per header file. Use short, systematic names.
Make these groups sub-groups of a few top-level groups, roughly
corresponding to the different logical parts of the API (core, impl,
stream, filter, spa, utilities).
Subdirectories buffer, control, debug, monitor, pod, support and utils, others
are still missing. Headers are grouped either per subdirectory (e.g. buffer/
gets added to group spa_buffer) or per-file (e.g. spa_json is a separate
group), whatever seemed like the most sensible approach.
