docs: scripts/sphinx-pre-install: suggest latexmk for building pdf

The usage of latexmk improves the PDF output, as it re-run
xelatex when it detects the need, in order to properly generate
indexes and cross-references.

As this is not a mandatory requirement, only suggest its
addition.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>