docs/conf.py: Configure the 'alabaster' theme

Add the 'navigation' bar to the sidebar, which for some
reason is not enabled by default. Remove 'relations', which
is effectively disabled anyway and isn't useful for us.

This requires that we mandate having at least Sphinx 1.3,
where the theme was added.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-id: 20190305172139.32662-6-peter.maydell@linaro.org
Message-id: 20190228145624.24885-6-peter.maydell@linaro.org