bpo-43795: Note Stable ABI PEP in What's New (GH-26479) (GH-26603)

(cherry picked from commit 257e400)
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>

Doc/whatsnew/3.10.rst

@@ -1798,6 +1798,16 @@ Build Changes
 C API Changes
 =============
 
+PEP 652: Maintaining the Stable ABI
+-----------------------------------
+
+The Stable ABI (Application Binary Interface) for extension modules or
+embedding Python is now explicitly defined.
+:ref:`stable` describes C API and ABI stability guarantees along with best
+practices for using the Stable ABI.
+
+(Contributed by Petr Viktorin in :pep:`652` and :issue:`43795`.)
+
 New Features
 ------------