Add more top-level references to XRC

Try to make XRC existence more discoverable.

docs/doxygen/overviews/helloworld.h

@@ -15,6 +15,10 @@ This page shows a very simple wxWidgets program that can be used as a skeleton
 for your own code. While it does nothing very useful, it introduces a couple of
 important concepts and explains how to write a working wxWidgets application.
 
+Note that this simple example creates the UI entirely from C++ code which is
+fine for a simple example, but more realistic examples will typically define
+their UI at least partially in @c overview_xrc "XRC resource files".
+
 First, you have to include wxWidgets' header files, of course. This can be done
 on a file by file basis (such as @c wx/window.h) or using one global include
 (@c wx/wx.h) which includes most of the commonly needed headers (although not