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

doc/input-filter.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+# -*- mode: python; coding: utf-8; eval: (blacken-mode); -*-
+r"""
+Doxygen input filter that:
+
+- adds \privatesection to all files
+- removes macros
+- parses pulse_module_options and substitutes it into @pulse_module_options@
+
+This is used for .c files, and causes Doxygen to not include
+any symbols from them, unless they also appeared in a header file.
+
+The Pulse module option parsing is used in documentation of Pulseaudio modules.
+"""
+import sys
+import re
+import os
+
+
+def main():
+    with open(sys.argv[1], "r") as f:
+        text = f.read()
+
+    text = re.sub("#define.*", "", text)
+
+    m = re.search(
+        r"static const char[* ]*const pulse_module_options\s+=\s+(.*?\")\s*;\s*$",
+        text,
+        re.M | re.S,
+    )
+    if m:
+        res = []
+        for line in m.group(1).splitlines():
+            m = re.match(r"\s*\"\s*([a-z0-9_]+)\s*=\s*(.*)\"\s*$", line)
+            if m:
+                name = m.group(1)
+                value = m.group(2).strip().strip("<>")
+                res.append(f"- `{name}`: {value}")
+
+        res = "\n * ".join(res)
+        text = text.replace("@pulse_module_options@", res)
+
+    print("/** \\privatesection */")
+    print(text)
+
+
+if __name__ == "__main__":
+    main()