Fix wrong @subpage usage on the main manual page

Using @subpage multiple times with the pages we wanted to just link to
resulted in them appearing twice in the tree shown in the CHM file.

Just use @ref when a link is wanted instead.

docs/doxygen/mainpages/manual.h

@@ -23,10 +23,10 @@ developing native cross-platform GUI applications in C++!
 
 @n
 
-If you are new to wxWidgets, please start with the @subpage page_introduction
-and follow with the @subpage page_topics, with maybe a look at
-@subpage page_samples as you go. If you are already familiar with wxWidgets,
-please read about @subpage overview_changes_since28 "the changes" in the latest
+If you are new to wxWidgets, please start with the @ref page_introduction
+and follow with the @ref page_topics, with maybe a look at
+@ref page_samples as you go. If you are already familiar with wxWidgets,
+please read about @ref overview_changes_since28 "the changes" in the latest
 version compared to 2.8 series. And you can also follow the links in the
 reference section or jump directly to the
 <a href="classes.html">alphabetical list of classes</a> to find out more about