tools/docs: sphinx-* break documentation bulds on openSUSE

tools/docs: sphinx-* break documentation bulds on openSUSE

Before this patch, building htmldocs on opensuseLEAP works
fine:

    # make htmldocs
    Available Python versions:
      /usr/bin/python3.11

    Python 3.6.15 not supported. Changing to /usr/bin/python3.11
    Python 3.6.15 not supported. Changing to /usr/bin/python3.11
    Using alabaster theme
    Using Python kernel-doc

    ...

As the logic detects that Python 3.6 is too old and recommends
intalling python311-Sphinx. If installed, documentation builds
work like a charm.

Yet, some develpers complained that running python3.11 instead
of python3 should not happen. So, let's break the build to make
them happier:

    $ make htmldocs
    Python 3.6.15 not supported. Bailing out
    You could run, instead:
      /usr/bin/python3.11 tools/docs/sphinx-build-wrapper htmldocs \
        --sphinxdirs=. --conf=conf.py --builddir=Documentation/output --theme= --css= \
        --paper=

    Python 3.6.15 not supported. Bailing out
    make[2]: *** [Documentation/Makefile:76: htmldocs] Error 1
    make[1]: *** [Makefile:1806: htmldocs] Error 2
    make: *** [Makefile:248: __sub-make] Error 2

It should be noticed that:

1. after this change, sphinx-pre-install needs to be called
   by hand:

    $ /usr/bin/python3.11 tools/docs/sphinx-pre-install
    Detected OS: openSUSE Leap 15.6.
    Sphinx version: 7.2.6

    All optional dependencies are met.
    Needed package dependencies are met.

2. sphinx-build-wrapper will auto-detect python3.11 and
   suggest a way to build the docs using the parameters passed
   via make variables. In this specific example:

   /usr/bin/python3.11 tools/docs/sphinx-build-wrapper htmldocs --sphinxdirs=. --conf=conf.py --theme= --css= --paper=

3. As this needs to be executed outside docs Makefile, it won't run
   the validation check scripts nor build Rust documentation if
   enabled, as the extra scripts are part of the docs Makefile.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Message-ID: <0635c311295300e9fb48c0ea607e2408910036e3.1758196090.git.mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>