docs: Makefile: use latexmk if available

In the past, Sphinx was generating a LaTex Makefile that would
run xelatex 3 times. Running it multiple times is needed in order
to make the indexes right.

However, newer versions of it runs it just once, as it expects
the machine to use the "latexmk" build, with automatically
detects the need for rebuilds.

So, add a logic at the Makefile in order to detect if latexmk
is installed. If so, it will call it.

As an additional bonus, the output of latexmk is a little bit
better, making easier to identify build problems.

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

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