[3.12] Docs: Use cross-reference to `os.uname` in `sysconfig.get_platform` (GH-122083) (#122172)

Docs: Use cross-reference to `os.uname` in `sysconfig.get_platform` (GH-122083)
(cherry picked from commit 498cb6dff10f97fa3d348a4c0ad9374d14af3312)

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>