doc: make all manpages with Doxygen

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.

doc/dox/programs/pw-loopback.1.md

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+\page page_man_pw-loopback_1 pw-loopback
+
+PipeWire loopback client
+
+# SYNOPSIS
+
+**pw-loopback** \[*options*\]
+
+# DESCRIPTION
+
+The *pw-loopback* program is a PipeWire client that uses the PipeWire
+loopback module to create loopback nodes, with configuration given via
+the command-line options.
+
+# OPTIONS
+
+\par -h | \--help
+Show help.
+
+\par -r | \--remote=NAME
+The name of the *remote* instance to connect to. If left unspecified, a
+connection is made to the default PipeWire instance.
+
+\par -n | \--name=NAME
+Name of the loopback node
+
+\par -g | \--group=NAME
+Name of the loopback node group
+
+\par -c | \--channels=NUMBER
+Number of channels to provide
+
+\par -m | \--channel-map=MAP
+Channel map (default `[ FL, FR ]`)
+
+\par -l | \--latency=LATENCY
+Desired latency in ms
+
+\par -d | \--delay=DELAY
+Added delay in seconds (floating point allowed)
+
+\par -C | \--capture=TARGET
+Target device to capture from
+
+\par -P | \--playback=TARGET
+Target device to play to
+
+\par \--capture-props=PROPS
+Wanted properties of capture node (in JSON)
+
+\par \--playback-props=PROPS
+Wanted properties of capture node (in JSON)
+
+# AUTHORS
+
+The PipeWire Developers <$(PACKAGE_BUGREPORT)>;
+PipeWire is available from <$(PACKAGE_URL)>
+
+# SEE ALSO
+
+\ref page_man_pipewire_1 "pipewire(1)",
+\ref page_man_pw-cat_1 "pw-cat(1)",
+**pactl(1)**
+
+Other ways to create loopback nodes are adding the loopback module in
+the configuration of a PipeWire daemon, or loading the loopback module
+using Pulseaudio commands (`pactl load-module module-loopback ...`).