buildroot

docs/website/support: clarify the bugtracker is for bugs only

The bugtracker is intended to be used for bug reports only, so clarify
it's not meant to be used for "any problem".

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>