docs: use --rebuild-types to autogenerate the file

We actually don't need the includes in the types file. Additionally this auto-
matically handles the optional parts (fluidsynth).
osc-synth: remove some defines from the header

Two are not actually needed and one can be made private.
childbin,tempo: fix the interface docs

Expose the interface structs.
make: remove whitespace
docs: reshuffle the sections file to be closer to the autogenerated version

Keep entries in the sections file in alphabetical order. Mark some headers as internal.
docs: add title and include tags to sections docs