docs: fix up reference docs a bit

      - require GTK-Doc 1.19
      - remove sgml mode
      - automatically generate gi.types (needs GTK-Doc 1.19)
      - fix https://bugzilla.gnome.org/show_bug.cgi?id=700025
[WIP] - rearange sections a bit
[WIP] - add gi-building, gi-programming sections
[WIP] - mark missing docs with TODO, which is only marginaly
        better than nothing but at least can be grepped :)

https://bugzilla.gnome.org/show_bug.cgi?id=571648