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/VisualShaderNodeOuterProduct.xml

@@ -8,9 +8,6 @@
 	</tutorials>
 	<methods>
 	</methods>
-	<members>
-		<member name="default_input_values" type="Array" setter="_set_default_input_values" getter="_get_default_input_values" override="true" default="[ 0, Vector3( 0, 0, 0 ), 1, Vector3( 0, 0, 0 ) ]" />
-	</members>
 	<constants>
 	</constants>
 </class>