doc: Misc updates for AnimationNode* and others

- Add some missing descriptions.
- Add links to tutorials for ARVR and AnimationTree.
- Style fixes.
- Engine changes:
  * Make `AnimationNodeTransition.input_<number>` properties internal
    so that they don't appear in the docs. They still appear in the
    inspector based on the actual number of inputs requested.
  * Drop unimplemented `CPUParticles.flatness`. It's only used for 3D
    particles in `ParticlesMaterial`, and thus only relevant for
    `CPUParticles3D`.

doc/classes/AnimationNodeAdd2.xml

@@ -7,6 +7,7 @@
 		A resource to add to an [AnimationNodeBlendTree]. Blends two animations additively based on an amount value in the [code][0.0, 1.0][/code] range.
 	</description>
 	<tutorials>
+		<link>https://docs.godotengine.org/en/latest/tutorials/animation/animation_tree.html</link>
 	</tutorials>
 	<methods>
 	</methods>