Add link to What's New

Author: encukou

Doc/whatsnew/3.15.rst

@@ -502,10 +502,6 @@ specifically:
   purpose in :pep:`793`, with a new :c:type:`PySlot` structure
   introduced in :pep:`820`.
 
-The reference documentation for these features is complete, but currently
-aimed at early adopters.
-A migration guide is planned for an upcoming beta release.
-
 Note that Stable ABI does not offer all the functionality that CPython
 has to offer.
 Extensions that cannot switch to ``abi3t`` should continue to build for
@@ -521,6 +517,9 @@ If not using a build tool -- or when writing such a tool -- you can select
 ``abi3t`` by setting the macro :c:macro:`!Py_TARGET_ABI3T` as discussed
 in :ref:`abi3-compiling`.
 
+A practical :ref:`migration guide <abi3t-migration-howto>` for switching to
+``abi3t`` is available.
+
 .. seealso:: :pep:`803` for further details.