[3.12] gh-130160: use `.. program::` directive for documenting `ensurepip` CLI (gh-130253) (gh-130258)

gh-130160: use `.. program::` directive for documenting `ensurepip` CLI (gh-130253)
(cherry picked from commit 8cd7f8bf8d6b62f30a8bba1f968eb0c593a8781a)

Co-authored-by: Kanishk Pachauri <itskanishkp.py@gmail.com>