Port from gnome-doc-utils to yelp

Ditch gnome-doc-utils which is obsolete now in favor of yelp
Thanks to David King for the help.

https://bugzilla.gnome.org/show_bug.cgi?id=699317
doc: include examples

Add the .c examples in the yelp documentation

https://bugzilla.gnome.org/show_bug.cgi?id=699317