This splits the manual into parts and groups the D-Bus interfaces into chapters
by the object class. It looks considerably better.

Turn the it into a chapter with refentries to better fit the structure
of the table of contects. Also, include the properties in the index.

I guess the daemon and its configuration first makes most sense.

Fixes: b42f780e045c69e6b2c350dcd6de760c673a78e0
Fixes: d252a99fa25359e3795abcacfae08c76dc5386b8

Building the man pages via xsltproc requires "docbook.xsl"
which is part of docbook.

Previously, we would build the man pages solely based on
"--enable-introspection", which checks for the presence of
xsltproc, but not docbook. This can lead to build failure
when docbook is not available, but "--enable-introspection"
is given.

Instead of adding yet another configure option to fine-tune
and say "--with-docbook --disable-gtk-doc", just simplify it.

Now, documentation (both man pages and setting docs) will be generated
with "--enable-gtk-doc" and "--enable-introspection".
If the documentation is not about to be generated, pre-generated docs
will be installed if they are available. That is commonly the case
with a source tarball, but not with a git checkout.
Finally, if documentation is nither generated nor pre-generated,
no documentation will be installed *duh*.

This removes the possibility to treat man pages separate from settings
docs. Now you either generate both, install both pre-generated, or don't
get any of them.

https://bugzilla.gnome.org/show_bug.cgi?id=778551