scripts: sphinx-pre-install: Add pyyaml hint to other distros

Extend commit 84b4cc8189f2 ("docs: scripts: sphinx-pre-install: Fix
building docs with pyyaml package") and add pyyaml as an optional
package to Mageia, ArchLinux, and Gentoo.

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.

Signed-off-by: Thorsten Blum <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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