Add reference to freethreading-stable-abi-howto

Author: clin1234

Doc/howto/index.rst

@@ -60,6 +60,7 @@ Advanced development:
 * :ref:`curses-howto`
 * :ref:`freethreading-python-howto`
 * :ref:`freethreading-extensions-howto`
+* :ref:`freethreading-stable-abi-howto`
 * :ref:`isolating-extensions-howto`
 * :ref:`python_2.3_mro`
 * :ref:`socket-howto`