docs: responsive web design website

This patch migrate the layout to a responsive web design one.

* Fixing the layout of all pages to be fully responsive
* Adding panels around "Recent commints" and "Recent discussion"
* Adding fonts for glyphs in buttons

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