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

Documentation/doc-guide/sphinx.rst

@@ -131,6 +131,29 @@ It supports two optional parameters:
 ``--no-virtualenv``
 	Use OS packaging for Sphinx instead of Python virtual environment.
 
+Installing Sphinx Minimal Version
+---------------------------------
+
+When changing Sphinx build system, it is important to ensure that
+the minimal version will still be supported. Nowadays, it is
+becoming harder to do that on modern distributions, as it is not
+possible to install with Python 3.13 and above.
+
+Testing with the lowest supported Python version as defined at
+Documentation/process/changes.rst can be done by creating
+a venv with it with, and install minimal requirements with::
+
+	/usr/bin/python3.9 -m venv sphinx_min
+	. sphinx_min/bin/activate
+	pip install -r Documentation/sphinx/min_requirements.txt
+
+A more comprehensive test can be done by using:
+
+	scripts/test_doc_build.py
+
+Such script create one Python venv per supported version,
+optionally building documentation for a range of Sphinx versions.
+
 
 Sphinx Build
 ============

Documentation/sphinx/min_requirements.txt

@@ -0,0 +1,10 @@
+alabaster >=0.7,<0.8
+docutils>=0.15,<0.18
+jinja2>=2.3,<3.1
+PyYAML>=5.1,<6.1
+Sphinx==3.4.3
+sphinxcontrib-applehelp==1.0.2
+sphinxcontrib-devhelp==1.0.1
+sphinxcontrib-htmlhelp==1.0.3
+sphinxcontrib-qthelp==1.0.2
+sphinxcontrib-serializinghtml==1.1.4