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).
Add more info to the main SPA page and split the design vs plugin pages up,
together with some more documentation to ideally lower make this easier to
understand on a glance.
Most of the actual plugin loading documentation are unmodified.