Add more top-level references to XRC

Try to make XRC existence more discoverable.

docs/doxygen/mainpages/introduction.h

@@ -41,8 +41,9 @@ Compared to the other similar libraries, wxWidgets is:
 3. Open source and free for use in both open source and commercial projects.
 
 
-wxWidgets provides a simple, easy to learn, yet very rich API. It is also
-mature and stable, and the applications written using wxWidgets 2.0
+wxWidgets provides a simple, easy to learn, yet very rich API, while also
+proposing a simple way to define application UI in the external XML files.
+It is also mature and stable, and the applications written using wxWidgets 2.0
 pre-releases almost 20 years ago can still be built today with wxWidgets 3
 almost unchanged. wxWidgets has a large, active and friendly community of
 people, including both the users and developers of the library. It is also
@@ -59,7 +60,7 @@ paragraphs, but here are some of the benefits:
 @li Extensive documentation (almost 200,000 lines of it).
 @li Straightforward API.
 @li Simple but powerful layout system.
-@li Run-time loadable or compile-time embeddable resources.
+@li Run-time loadable or compile-time embeddable resources for UI definition.
 @li Flexible event system.
 @li All the usual and quite a few of more rare GUI controls.
 @li And also all the standard dialogs.

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

docs/doxygen/overviews/roughguide.h

@@ -19,7 +19,8 @@ main top-level window.
 
 This window can be a wxFrame or a wxDialog and may contain one or more
 instances of classes such as wxPanel, wxSplitterWindow or other windows and
-controls.
+controls. These windows can be created from C++ code or loaded from resource
+definitions in @ref overview_xrc "XRC format".
 
 A frame can have a wxMenuBar, a wxToolBar, a wxStatusBar, and a wxIcon for when
 the frame is iconized.