Add instruction to bake navigation BakePolygon

tutorials/navigation/navigation_introduction_2d.rst

@@ -94,7 +94,8 @@ It uses the NavigationServer2D and a NavigationAgent2D for path movement.
 
    .. image:: img/nav_2d_min_setup_step1.png
 
-#. Define the moveable navigation area with the NavigationPolygon draw tool.
+#. Define the moveable navigation area with the NavigationPolygon draw tool. Then click
+   the `Bake NavigationPolygon`` button on the toolbar.
 
    .. image:: img/nav_2d_min_setup_step2.png