docs: Fix links to symbols outside the allowed namsepace

Use markdown links for anything that the docs don't depend upon,
like gtk, pixbuf, etc

gio/gmenumodel.c

@@ -103,7 +103,7 @@
  * identifies an action in an action group. Which action group(s) provide
  * actions depends on the context in which the menu model is used.
  * E.g. when the model is exported as the application menu of a
- * [class@Gtk.Application], actions can be application-wide or window-specific
+ * [GtkApplication](https://docs.gtk.org/gtk4/class.Application.html), actions can be application-wide or window-specific
  * (and thus come from two different action groups). By convention, the
  * application-wide actions have names that start with `app.`, while the
  * names of window-specific actions start with `win.`.