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/PhysicsServer.xml

@@ -1315,7 +1315,7 @@
 			If [code]true[/code], the Hinge has a maximum and a minimum rotation.
 		</constant>
 		<constant name="HINGE_JOINT_FLAG_ENABLE_MOTOR" value="1" enum="HingeJointFlag">
-			If [code]true[/code], a motor turns the Hinge
+			If [code]true[/code], a motor turns the Hinge.
 		</constant>
 		<constant name="SLIDER_JOINT_LINEAR_LIMIT_UPPER" value="0" enum="SliderJointParam">
 			The maximum difference between the pivot points on their X axis before damping happens.