[3.15] gh-146636: Add Free-threaded Stable ABI migration guide (GH-150580) by miss-islington · Pull Request #150844 · python/cpython

…GH-150580)

(cherry picked from commit c720e18)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Charlie Lin <tuug@gmx.us>
Co-authored-by: da-woods <dw-git@d-woods.co.uk>
Co-authored-by: Stan Ulbrych <stan@python.org>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

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