devel-docs: add URLs to babl and GEGL gi-docgen documentation.

Note that these links are not valid yet. For now, documentations are only in
testing:
* babl: https://testing.developer.gimp.org/api/babl/
* GEGL: https://testing.developer.gimp.org/api/gegl/

The main links will appear next time we merge testing into the main developer
website, or at the next release of GIMP.

devel-docs/reference/gimp-ui/urlmap.js

@@ -1,5 +1,7 @@
 // A map between namespaces and base URLs for their online gi-docgen documentation
 baseURLs = [
+  [ 'Babl',      'https://developer.gimp.org/api/babl/' ],
+  [ 'Gegl',      'https://developer.gimp.org/api/gegl/' ],
   [ 'GLib',      'https://docs.gtk.org/glib/' ],
   [ 'GObject',   'https://docs.gtk.org/gobject/' ],
   [ 'Gdk',       'https://docs.gtk.org/gdk3/' ],

devel-docs/reference/gimp/urlmap.js

@@ -1,5 +1,7 @@
 // A map between namespaces and base URLs for their online gi-docgen documentation
 baseURLs = [
+  [ 'Babl',      'https://developer.gimp.org/api/babl/' ],
+  [ 'Gegl',      'https://developer.gimp.org/api/gegl/' ],
   [ 'GLib',      'https://docs.gtk.org/glib/' ],
   [ 'GObject',   'https://docs.gtk.org/gobject/' ],
   [ 'Gdk',       'https://docs.gtk.org/gdk3/' ],