Filled tutorial field in most relevent classes.

Added tutorial display in doc.

doc/classes/World2D.xml

@@ -7,6 +7,7 @@
 		Class that has everything pertaining to a 2D world. A physics space, a visual scenario and a sound space. 2D nodes register their resources into the current 2D world.
 	</description>
 	<tutorials>
+	http://docs.godotengine.org/en/3.0/learning/features/physics/ray-casting.html
 	</tutorials>
 	<demos>
 	</demos>