docs: conf.py: bump minimum Sphinx version requirement

extlinks captions support using %s substitution but only from sphinx
4.0 onwards. Anything before does not support the %s substitution.
However, this will be a requirement in sphinx 6.0 and later, and sphinx
5.0 already shows a warning telling us that.

Since the build is made with -W flag which fails it if any warning is
printed, a fix is required.

Sphinx 4.0.0 was released two years ago. Unfortunately, Debian packages
are out-of-date and only the latest Ubuntu has recent enough packages.
Therefore, pip installation should be recommended now instead and the
minimum version for building the docs bumped to 4.0.

Cc: Quentin Schulz <foss+yocto@0leil.net>
(From yocto-docs rev: ab45a7a40484a2d8e604cb45549037c526ec61cb)

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/conf.py

@@ -53,7 +53,7 @@ author = 'The Linux Foundation'
 # -- General configuration ---------------------------------------------------
 
 # Prevent building with an outdated version of sphinx
-needs_sphinx = "3.1"
+needs_sphinx = "4.0"
 
 # to load local extension from the folder 'sphinx'
 sys.path.insert(0, os.path.abspath('sphinx'))