[3.14] gh-134160: Split extension module init from PyModule docs; emphasize multi-phase init (GH-135126) (#135470)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>