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`.

modules/websocket/doc_classes/WebSocketPeer.xml

@@ -66,7 +66,7 @@
 			<return type="bool">
 			</return>
 			<description>
-				Returns [code]true[/code] if the last received packet was sent as a text payload. See [enum WriteMode]
+				Returns [code]true[/code] if the last received packet was sent as a text payload. See [enum WriteMode].
 			</description>
 		</method>
 	</methods>