Fixed 2 'Introduction' in Navigation tutorial (#6269)

* Fixed 2 'Introduction' in Navigation tutorail

* removed intro para from main navigation and reverted to original location

tutorials/navigation/navigation_introduction_2d.rst

@@ -1,14 +1,10 @@
 .. _doc_navigation_overview_2d:
 
-Introduction
-===================
-
-Godot provides multiple objects, classes and servers to facilitate grid-based or mesh-based navigation
-and pathfinding for 2D and 3D games. The following section provides a quick overview over all available
-navigation related objects in Godot for 2D scenes and their primary use.
-
 2D Navigation Overview
-----------------------
+======================
+
+Godot provides multiple objects, classes and servers to facilitate grid-based or mesh-based navigation and pathfinding for 2D and 3D games. 
+The following section provides a quick overview over all available navigation related objects in Godot for 2D scenes and their primary use.
 
 Godot provides the following objects and classes for 2D navigation:
 

tutorials/navigation/navigation_introduction_3d.rst

@@ -1,14 +1,11 @@
 .. _doc_navigation_overview_3d:
 
-Introduction
-===================
-
-Godot provides multiple objects, classes and servers to facilitate grid-based or mesh-based navigation
-and pathfinding for 2D and 3D games. The following section provides a quick overview over all available
-navigation related objects in Godot for 3D scenes and their primary use.
 
 3D Navigation Overview
-----------------------
+======================
+
+Godot provides multiple objects, classes and servers to facilitate grid-based or mesh-based navigation and pathfinding for 2D and 3D games. 
+The following section provides a quick overview over all available navigation related objects in Godot for 3D scenes and their primary use.
 
 Godot provides the following objects and classes for 3D navigation: