Add AstarGrid2D headline to 2D Navigation Overview (#10135)

* Add AstarGrid2D headline to 2D Navigation Overview

Resolves #9668 by adding in the request description.

This PR adds a short description for AStarGrid2D of its function and use case that was previously missing in the 2D navigation overview page.

tutorials/navigation/navigation_introduction_2d.rst

@@ -13,6 +13,11 @@ Godot provides the following objects and classes for 2D navigation:
 
     The AStar2D class is best suited for cell-based 2D gameplay that does not require actors to reach any possible position within an area but only predefined, distinct positions.
 
+- :ref:`AstarGrid2D<class_AstarGrid2D>`
+    ``AstarGrid2D``  is a variant of AStar2D that is specialized for partial 2D grids. 
+
+    AstarGrid2D is simpler to use when applicable because it doesn't require you to manually create points and connect them together.
+
 - :ref:`NavigationServer2D<class_NavigationServer2D>`
     ``NavigationServer2D`` provides a powerful server API to find the shortest path between two positions on an area defined by a navigation mesh.