docs: sphinx: add a file with the requirements for lowest version

docs: sphinx: add a file with the requirements for lowest version

Those days, it is hard to install a virtual env that would
build docs with Sphinx 3.4.3, as even python 3.13 is not
compatible anymore with it.

	/usr/bin/python3.9 -m venv sphinx_3.4.3
	. sphinx_3.4.3/bin/activate
	pip install -r Documentation/sphinx/min_requirements.txt

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/e38a44ee64ebfa37eac5f64e47af51c7ac051d5a.1750571906.git.mchehab+huawei@kernel.org