Add introduction to 2D index page

Author: tetrapod00

tutorials/2d/index.rst

@@ -1,5 +1,14 @@
 :allow_comments: False
 
+Godot includes a dedicated 2D renderer and 2D physics engine, as well as
+2D-specific features like tilemaps, particles, and animation systems. This
+section covers most 2D-specific topics in Godot.
+
+For 2D topics not covered in this section, see also :ref:`doc_2d_skeletons` and
+:ref:`doc_navigation_overview_2d`. For using physics in 2D, see
+:ref:`doc_physics_index`. There is also a step-by-step tutorial on creating a 2D
+game in :ref:`doc_your_first_2d_game`.
+
 2D
 ==
 

tutorials/physics/index.rst

@@ -1,5 +1,7 @@
 :allow_comments: False
 
+.. _doc_physics_index:
+
 Physics
 =======