[RFC] doc: updating doxy-api-index.md and doxy-api.conf.in

[Marat Khalili]

I noticed that a few header files contain docstring-like comments but are not
present in doc/api/doxy-api-index.md and/or (fewer) in doc/api/doxy-api.conf.in
There seem to be no checks anywhere in the process that would make sure these
index files are updated. Do we even care, or is it unsupported legacy? If we do
care, would a check or an automatic generation fix be more promising?

With Best Regards,
Marat