doc: use XeLaTeX as TeX typesetting engine

XeLaTeX is a TeX typesetting engine that extends LaTeX with support for
Unicode and modern font technologies such as OpenType.

It is included in all the LaTeX distro recommended for Zephyr so it is
safe to assume it is available to the user interested in building the
PDF documentation locally

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>