doc: generate man-pages also for pipewire modules

Use pandoc + some processing to convert Doxygen html output to man
pages.

Requires pandoc & python for building.

Generates manpages: libpipewire-modules.7, libpipewire-module-*.7

man/libpipewire-modules.7.rst.in

@@ -0,0 +1,56 @@
+libpipewire-modules
+###################
+
+----------------
+PipeWire modules
+----------------
+
+:Manual section: 7
+:Manual group: PipeWire
+
+DESCRIPTION
+===========
+
+A PipeWire module is effectively a PipeWire client running inside
+``pipewire(1)`` which can host multiple modules. Usually modules are
+loaded when they are listed in the configuration files. For example
+the default configuration file loads several modules:
+
+::
+
+    context.modules = [
+        ...
+        # The native communication protocol.
+        {   name = libpipewire-module-protocol-native }
+    
+        # The profile module. Allows application to access profiler
+        # and performance data. It provides an interface that is used
+        # by pw-top and pw-profiler.
+        {   name = libpipewire-module-profiler }
+    
+        # Allows applications to create metadata objects. It creates
+        # a factory for Metadata objects.
+        {   name = libpipewire-module-metadata }
+    
+        # Creates a factory for making devices that run in the
+        # context of the PipeWire server.
+        {   name = libpipewire-module-spa-device-factory }
+        ...
+    ]
+
+KNOWN MODULES
+=============
+
+- @LIBPIPEWIRE_MODULES@
+
+AUTHORS
+=======
+
+The PipeWire Developers <@PACKAGE_BUGREPORT@>; PipeWire is available from @PACKAGE_URL@
+
+SEE ALSO
+========
+
+``pipewire(1)``,
+``pipewire.conf(5)``,
+