Add a link to the notifications tutorial page in the object class reference

doc/classes/Object.xml

@@ -30,6 +30,7 @@
 	</description>
 	<tutorials>
 		<link title="When and how to avoid using nodes for everything">$DOCS_URL/tutorials/best_practices/node_alternatives.html</link>
+		<link title="Object notifications">$DOCS_URL/tutorials/best_practices/godot_notifications.html</link>
 	</tutorials>
 	<methods>
 		<method name="_get" qualifiers="virtual">