Rename animation_2d page to animation

Closes #4136

tutorials/animation/index.rst

@@ -5,7 +5,7 @@ Animation
    :maxdepth: 1
    :name: toc-learn-features-animation
 
-   introduction_2d
+   introduction
    cutout_animation
    2d_skeletons
    animation_tree

tutorials/animation/introduction_2d.rst → tutorials/animation/introduction.rst

@@ -1,10 +1,7 @@
 .. _doc_introduction_2d_animation:
 
-Introduction to the 2D animation features
-=========================================
-
-Overview
---------
+Introduction to the animation features
+======================================
 
 The :ref:`class_AnimationPlayer` node allows you to create anything
 from simple to complex animations.