docs/website: improve website speed

docs/website: improve website speed

Currently the buildroot website is not leveraging common
techniques to have a better loading speed (browser caching,
gzip compression, deflating).
This commit provides an .htaccess files with all the needed to
enable mentioned features. This .htaccess only works if the
webserver has deflate, expires and headers modules enabled.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>