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 <[email protected]>
Reviewed-by: Vegard Nossum <[email protected]>
Tested-by: Vegard Nossum <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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