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

libcroesor/docs/libcroesor-docs.sgml

 <?xml version="1.0"?>
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
-               "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd"
-[
-  <!ENTITY % local.common.attrib "xmlns:xi  CDATA  #FIXED 'http://www.w3.org/2003/XInclude'">
-  <!ENTITY % gtkdocentities SYSTEM "xml/gtkdocentities.ent">
-  %gtkdocentities;
-]>
+               "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
 <book id="index" xmlns:xi="http://www.w3.org/2003/XInclude">
   <bookinfo>
-    <title>&package_name; Reference Manual</title>
-    <releaseinfo>
-      for &package_string;.
-      The latest version of this documentation can be found on-line at
-      <ulink role="online-location" url="http://[SERVER]/&package_name;/index.html">http://[SERVER]/&package_name;/</ulink>.
-    </releaseinfo>
+    <title>libcroesor Reference Manual</title>
   </bookinfo>
 
+  <xi:include href="overview.xml"/>
   <chapter>
-    <title>[Insert title here]</title>
+    <title>libcroesor APIs</title>
     <xi:include href="xml/backend.xml"/>
     <xi:include href="xml/backend-manager.xml"/>
     <xi:include href="xml/clock.xml"/>