docs: scripts: sphinx-pre-install: Fix building docs with pyyaml package

The Python module pyyaml is required to build the docs, but it is only
listed in Documentation/sphinx/requirements.txt and is therefore missing
when Sphinx is installed as a package and not via pip/pypi.

Add pyyaml as an optional package for multiple distros to fix building the
docs if you prefer to install Sphinx as a package.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Tested-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240208205550.984-1-thorsten.blum@toblux.com