Use (fixed-up) Doxygen manpage output for all program & module manpages.
This also allows formatting the manual pages properly in the HTML docs.
The Markdown pages work properly only with Doxygen >= 1.9.7, older
versions put them to wrong place in the HTML docs.
Add (minimal) reference documentation for each pipewire-pulse module.
Add some preprocessing to substitute @pulse_module_options@ in docs from
PW_KEY_MODULE_USAGE so the module options don't need to be repeated.
Produce Doxygen docs + generate manpages pipewire-pulse-modules.7,
pipewire-pulse-module-*.7
