README: update instructions for installing vale and sphinx-lint

pip install won't work on recent Debian distros (and Debian-based ones
I'm assuming as well) and will suggest using pipx instead.

In any case, pip and pipx aren't actually that user-friendly to us for
the simple reason the instructions to build the documentation, c.f.
documentation/tools/host_packages_scripts/pip3_docs.sh, make use of a
virtual environment which doesn't have access to the host system's
Python modules (missing --system-site-packages). So you would need to
chose between building the docs and running vale/sphinx-lint but not
both at the same time.

Instead, update the instructions to use pipenv for stylecheck and
sphinx-lint make targets such that there's a setup in which one can
build the docs AND run those commands without doing some back and forth
between venvs or host packages and the venv.

Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Closes: https://lore.kernel.org/yocto-docs/959e65b6-e7eb-bcc3-992f-bed7276c8449@crashcourse.ca/
(From yocto-docs rev: fe46d5de41930a2762a9ad2543d50e0f4e10463b)

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Link: https://patch.msgid.link/20260821-pipenv-vale-sphinx-lint-v1-1-f108b7e3e739@cherry.de
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 9955b0f099b6b42a9750f0d544944a2d8e5a39b2)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Paul Barker <paul@pbarker.dev>

documentation/Pipfile

@@ -4,6 +4,8 @@ url = "https://pypi.org/simple"
 verify_ssl = true
 
 [dev-packages]
+sphinx-lint = "*"
+vale = "*"
 
 [packages]
 sphinx = "*"