gh-109975: Add links to py-free-threading.github.io (#123776)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: ngoldbaum

Doc/howto/free-threading-extensions.rst

@@ -270,3 +270,9 @@ Windows
 
 Due to a limitation of the official Windows installer, you will need to
 manually define ``Py_GIL_DISABLED=1`` when building extensions from source.
+
+.. seealso::
+
+   `Porting Extension Modules to Support Free-Threading
+   <https://py-free-threading.github.io/porting/>`_:
+   A community-maintained porting guide for extension authors.

Doc/whatsnew/3.13.rst

@@ -359,6 +359,10 @@ free-threaded build.
    :pep:`703` "Making the Global Interpreter Lock Optional in CPython"
    contains rationale and information surrounding this work.
 
+   `Porting Extension Modules to Support Free-Threading
+   <https://py-free-threading.github.io/porting/>`_: A community-maintained
+   porting guide for extension authors.
+
 
 .. _whatsnew313-jit-compiler: