DOC Add changelog for free-threaded support (scikit-learn#30360)

Author: lesteve

doc/whats_new/upcoming_changes/custom-top-level/30360.other.rst

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+Free-threaded CPython 3.13 support
+----------------------------------
+
+scikit-learn has preliminary support for free-threaded CPython, in particular
+free-threaded wheels are available for all of our supported platforms.
+
+Free-threaded (also known as nogil) CPython 3.13 is an experimental version of
+CPython 3.13 who aims at enabling efficient multi-threaded use cases by
+removing the Global Interpreter Lock (GIL).
+
+For more details about free-threaded CPython see `py-free-threading doc <https://py-free-threading.github.io>`_,
+in particular `how to install a free-threaded CPython <https://py-free-threading.github.io/installing_cpython/>`_
+and `Ecosystem compatibility tracking <https://py-free-threading.github.io/tracking/>`_.
+
+Feel free to try free-threaded on your use case and report any issues!
+
+By :user:`Loïc Estève <lesteve>` and many other people in the wider Scientific
+Python and CPython ecosystem, for example :user:`Nathan Goldbaum <ngoldbaum>`,
+:user:`Ralf Gommers <rgommers>`, :user:`Edgar Andrés Margffoy Tuay <andfoy>`.