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/BitmapFont.xml

@@ -2,7 +2,7 @@
 <class name="BitmapFont" inherits="Font" category="Core" version="3.2">
 	<brief_description>
 		Renders text using fonts under the [url=https://www.angelcode.com/products/bmfont/]BMFont[/url] format.
-		Handles files with the [code].fnt[/code] extension
+		Handles files with the [code].fnt[/code] extension.
 	</brief_description>
 	<description>
 		Renders text using [code]*.fnt[/code] fonts containing texture atlases. Supports distance fields. For using vector font files like TTF directly, see [DynamicFont].