Rename the Lights and shadows page to 3D lights and shadows (#3875)

This distinguishes it better from its 2D counterpart.

The filename has been left unchanged as to avoid breaking URLs.

tutorials/3d/lights_and_shadows.rst

@@ -1,7 +1,7 @@
 .. _doc_lights_and_shadows:
 
-Lights and shadows
-==================
+3D lights and shadows
+=====================
 
 Introduction
 ------------