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/GPUParticles2D.xml

@@ -8,7 +8,7 @@
 		Use the [code]process_material[/code] property to add a [ParticlesMaterial] to configure particle appearance and behavior. Alternatively, you can add a [ShaderMaterial] which will be applied to all particles.
 	</description>
 	<tutorials>
-		<link>https://docs.godotengine.org/en/latest/tutorials/2d/particle_systems_2d.html</link>
+		<link title="Particle systems (2D)">https://docs.godotengine.org/en/latest/tutorials/2d/particle_systems_2d.html</link>
 	</tutorials>
 	<methods>
 		<method name="capture_rect" qualifiers="const">