Add a "see also" link to the GDScript grammar in the GDScript section

The page is in the Development section, but it's not easy to discover.

(cherry picked from commit ea7814b25c0ccd0aac728d54aedad461ea3c868d)

getting_started/scripting/gdscript/gdscript_basics.rst

@@ -1142,7 +1142,7 @@ in other languages)::
           There is no need to call them explicitly when overloading them.
 
 
-Class Constructor
+Class constructor
 ^^^^^^^^^^^^^^^^^
 
 The class constructor, called on class instantiation, is named ``_init``. As

getting_started/scripting/gdscript/index.rst

@@ -12,3 +12,8 @@ GDScript
    static_typing
    warning_system
    gdscript_format_string
+
+.. seealso::
+
+    See :ref:`doc_gdscript_grammar` if you are interested in writing a third-party
+    tool that interacts with GDScript, such as a linter or formatter.