Add link to tool docs in gdscript docs.

The gdscript_basics docs give a pretty sparse description of the tool
keyword. I ended up figuring out a lot of it on my own before I finally
realized there was a more detailed page hidden elsewhere in the docs.

getting_started/scripting/gdscript/gdscript_basics.rst

@@ -1256,6 +1256,9 @@ placed at the top of the file::
     func _ready():
         print("Hello")
 
+
+See :ref:`doc_running_code_in_the_editor` for more information.
+
 .. warning:: Be cautious when freeing nodes with ``queue_free()`` or ``free()``
              in a tool script (especially the script's owner itself). As tool
              scripts run their code in the editor, misusing them may lead to