[3.14] gh-139707: Add docs for optional modules (GH-140171) (GH-141204)

(cherry picked from commit d2ce6d708a9eaac4e546744ca4da359ee6901ebc)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Emma Smith <emma@emmatyping.dev>
Co-authored-by: Author: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Victor Stinner <vstinner@python.org>