scripts/sphinx-pre-install: make it handle Sphinx versions

As we want to switch to a newer Sphinx version in the future,
add some version detected logic, checking if the current
version meets the requirement and suggesting upgrade it the
version is supported but too old.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>