Docs: add whatnew for free-threading

Doc/whatsnew/3.13.rst

@@ -99,6 +99,12 @@ New typing features:
 * :pep:`742`: :data:`typing.TypeIs` was added, providing more intuitive
   type narrowing behavior.
 
+Free-threading:
+
+* :pep:`703`: CPython 3.13 has experimental support for running with the
+  :term:`global interpreter lock` disabled when built with ``--disable-gil``.
+  See :ref:`Free-threaded CPython <free-threaded-cpython>` for more details.
+
 New Features
 ============
 
@@ -1052,6 +1058,30 @@ See :pep:`744` for more details.
 Tier 2 IR by Mark Shannon and Guido van Rossum.
 Tier 2 optimizer by Ken Jin.)
 
+.. _free-threaded-cpython:
+
+Free-threaded CPython
+=====================
+
+CPython will run with the :term:`global interpreter lock` disabled when
+configured using the ``--disable-gil`` option. This is an experimental feature
+and work is still ongoing: expect some bugs and a substantial single-threaded
+performance hit.
+
+* Use :func:`!sys._is_gil_enabled` to determine if the :term:`GIL` is enabled.
+
+* Use ``sysconfig.get_config_var("Py_GIL_DISABLED")`` to identify CPython
+  builds configured with ``--disable-gil``.
+
+C-API extensions need to be built specifically for the free-threaded build.
+
+* Extensions that support running with the :term:`GIL` disabled should use
+  the :c:data:`Py_mod_gil` slot. Extensions using single-phase init should use
+  :c:func:`PyUnstable_Module_SetGIL` to indicate whether they support running
+  with the GIL disabled.
+
+* pip 24.1b1 or newer is required to install packages with C extensions in the
+  free-threaded build.
 
 
 Deprecated