Add link titles for all links in the class reference

This makes them display in a nicer way in the editor help.
(The title will display instead of the full URL.)

doc/classes/AABB.xml

@@ -7,7 +7,7 @@
 		AABB consists of a position, a size, and several utility functions. It is typically used for fast overlap tests.
 	</description>
 	<tutorials>
-		<link>https://docs.godotengine.org/en/latest/tutorials/math/index.html</link>
+		<link title="Math tutorial index">https://docs.godotengine.org/en/latest/tutorials/math/index.html</link>
 	</tutorials>
 	<methods>
 		<method name="AABB">