Filled tutorial field in most relevent classes.

Added tutorial display in doc.

modules/gdscript/doc_classes/GDScript.xml

@@ -8,6 +8,7 @@
 		[method new] creates a new instance of the script. [method Object.set_script] extends an existing object, if that object's class matches one of the script's base classes.
 	</description>
 	<tutorials>
+	http://docs.godotengine.org/en/3.0/learning/scripting/gdscript/index.html
 	</tutorials>
 	<demos>
 	</demos>