docs/website: New website

This patch introduces a new buildroot website based on a modern flat
layout inspired by material design.

Each page of the old website is converted to the new css/layout, the
big bump is in main page which is completely rewritten. Content of
the old website is copied more or less verbatim, planning to add more
content in the future.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>