Switch back to Beautiful Hugo theme and tweak

The Simple Hugo theme is a bit too minimalistic (which was helpful during
conversion from a wiki, but less desirable as we look to go public with
the hugo site. Switch back to the Beautiful Hugo theme and tweak both the
theme and the config to get the site a little closer to Apertis branding.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>