Reference "Using an external text editor" in the "Visual Studio Code" page (#11158)

* Reference "Using an external text editor" in the "Visual Studio Code" page.

Right now, it says that the Visual Studio Code page is the page for
contributing to the engine, and refers the user to the C# guide to use
an external editor. This commit also includes the GDScript-specific page.

contributing/development/configuring_an_ide/visual_studio_code.rst

@@ -7,7 +7,8 @@ Visual Studio Code
 
     This documentation is for contributions to the game engine, and not using
     Visual Studio Code as a C# or GDScript editor. To code C# or GDScript in an external editor, see
-    :ref:`the C# guide to configure an external editor <doc_c_sharp_setup_external_editor>`.
+    :ref:`the C# guide to configure an external editor <doc_c_sharp_setup_external_editor>` or
+    :ref:`the GDScript guide to using an external text editor <doc_external_editor>`.
 
 `Visual Studio Code <https://code.visualstudio.com>`_ is a free cross-platform code editor
 by `Microsoft <https://microsoft.com>`_ (not to be confused with :ref:`doc_configuring_an_ide_vs`).