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/VisualShaderNodeTexture.xml

@@ -9,7 +9,6 @@
 	<methods>
 	</methods>
 	<members>
-		<member name="default_input_values" type="Array" setter="_set_default_input_values" getter="_get_default_input_values" override="true" default="[  ]" />
 		<member name="source" type="int" setter="set_source" getter="get_source" enum="VisualShaderNodeTexture.Source" default="0">
 		</member>
 		<member name="texture" type="Texture" setter="set_texture" getter="get_texture">