Add gtk-doc documentation

Since hotdoc support has been dropped, there has been no recommended API documentation tool.

We decided to go with gtk-doc and this MR updates the documentation of rhosydd to make it compatible with gtk-doc

It also makes sure that the -doc debian packages are built and that the documentation is installed at the right place to be used by devhelp.

https://phabricator.apertis.org/T8978

debian/changelog

+rhosydd (0.2022.1) apertis; urgency=medium
+
+  * doc: Update documentation and build it with GTK-doc
+
+ -- Detlev Casanova <detlev.casanova@collabora.com>  Thu, 11 Aug 2022 12:04:52 -0400
+
 rhosydd (0.2022.0) apertis; urgency=medium
 
   [ Akshay M ]