doc: add documentation for pipewire-pulse modules

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

man/pipewire-pulse-modules.7.rst.in

@@ -0,0 +1,43 @@
+pipewire-pulse-modules
+######################
+
+---------------------------
+PipeWire Pulseaudio modules
+---------------------------
+
+:Manual section: 7
+:Manual group: PipeWire
+
+DESCRIPTION
+===========
+
+PipeWire's Pulseaudio emulation implements several Pulseaudio modules.
+It only supports its own built-in modules, and cannot load external
+modules written for Pulseaudio.
+
+The built-in modules can be loaded using Pulseaudio client programs,
+for example `pactl load-module <module-name> <module-options>`. They
+can also added to `pipewire-pulse.conf`, typically by a drop-in file
+in `~/.config/pipewire/pipewire-pulse.conf.d/` containing the module
+name and its arguments
+
+::
+
+    pulse.cmd = [
+       { cmd = "load-module" args = "module-null-sink sink_name=foo" flags = [ ] }
+    ]
+
+KNOWN MODULES
+=============
+
+- @PIPEWIRE_PULSE_MODULES@
+
+AUTHORS
+=======
+
+The PipeWire Developers <@PACKAGE_BUGREPORT@>; PipeWire is available from @PACKAGE_URL@
+
+SEE ALSO
+========
+
+``pipewire-pulse(1)``,