Link to demos from within the class reference

(cherry picked from commit 439be614f4)

doc/classes/Timer.xml

@@ -8,6 +8,7 @@
 		[b]Note:[/b] To create an one-shot timer without instantiating a node, use [method SceneTree.create_timer].
 	</description>
 	<tutorials>
+		<link title="2D Dodge The Creeps Demo">https://godotengine.org/asset-library/asset/515</link>
 	</tutorials>
 	<methods>
 		<method name="is_stopped" qualifiers="const">