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/AnimationNodeBlendTree.xml

@@ -5,6 +5,7 @@
 	<description>
 	</description>
 	<tutorials>
+		<link>https://docs.godotengine.org/en/latest/tutorials/animation/animation_tree.html</link>
 	</tutorials>
 	<methods>
 		<method name="add_node">