scripts: sphinx-pre-install: provide both venv and package installs

As it is not a consensus about installing sphinx using venv, and
modern distributions are now shipping with Sphinx versions above
the minimal requirements to build the docs, provide both venv
and package install commands by default.

Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/7c99d985df204c73e3daafd5fd2f30a31269405d.1656756450.git.mchehab@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>