Update URLs library.gnome.org -> developer.gnome.org
docs: syntax highlighting for the code examples

In the sections Concepts, Tools and Tutorial.

https://bugzilla.gnome.org/show_bug.cgi?id=736914
docs: code example for enumeration types

I had to read the GTK+ code recently to have an example.

https://bugzilla.gnome.org/show_bug.cgi?id=736914
docs: fix indentation in gobject-docs.xml