docs: Remove clutter_stage_get_default()

A lot of the example code in the cookbook and the API reference still
uses the default stage — sometimes as if it were a non-default one,
which once again demonstrates how the default stage was a flawed concept
that just confused people.

doc/cookbook/examples/textures-crossfade.c

@@ -85,7 +85,7 @@ main (int argc, char *argv[])
   if (clutter_init (&argc, &argv) != CLUTTER_INIT_SUCCESS)
     return 1;
 
-  stage = clutter_stage_get_default ();
+  stage = clutter_stage_new ();
   clutter_stage_set_title (CLUTTER_STAGE (stage), "cross-fade");
   clutter_actor_set_size (stage, 400, 300);
   clutter_actor_show (stage);