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

Co-authored-by: Ken Jin <[email protected]>

Author: encukou

Doc/whatsnew/3.10.rst

@@ -1791,6 +1791,16 @@ 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
 ------------