Update using_tilemaps.rst

Make 'Introduction' a chapter of 'Using tilemaps' and not a main documentation entry

tutorials/2d/using_tilemaps.rst

@@ -4,7 +4,7 @@ Using tilemaps
 ~~~~~~~~~~~~~~
 
 Introduction
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+------------
 
 Tilemaps are a simple and quick way to make 2D game levels. Basically,
 you start with bunch of reference tiles (or pieces) that can be put in a