docs: use Markdown syntax for links

GTK-Doc >= 1.18 is required for Markdown support.
docs: improve a little the short descriptions

No need to say "Object which blabla". The "blabla" is sufficient.
docs: improve doc of GtkSourceView and GtkSourceBuffer
Improve API reference structure by grouping classes

The classes were just sorted alphabetically. The GtkSourceView class at
the end was not great, since it's the main class.

The features with only one class are grouped into "Others".