documentation: Prevent building documentation with an outdated version of sphinx

Building with a outdated version of Sphinx print warnings that does not
appear on up-to-date sphinx.

This patch prevent building the documentation with any version older
than 3.1 (First version to build without warnings in my tests)

See threads "documentation: Add a simple Sphinx extension to check its version"
    https://lists.yoctoproject.org/g/docs/topic/patch_documentation_add_a/79919516
and "toaster-manual: Fix a warning related to the code-block directive"
    https://lists.yoctoproject.org/g/docs/topic/patch_toaster_manual_fix_a/79656195

(From yocto-docs rev: 4de0f3dd4d5df0a0700f704a599bb41726d15a5f)

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/conf.py

@@ -33,6 +33,9 @@ author = 'The Linux Foundation'
 
 # -- General configuration ---------------------------------------------------
 
+# Prevent building with an outdated version of sphinx
+needs_sphinx = "3.1"
+
 # to load local extension from the folder 'sphinx'
 sys.path.insert(0, os.path.abspath('sphinx'))