Button Get GNOME 3 added on front page

A "Get GNOME 3" button was added to the frontpage which links directly
to download page. This makes it easier to get to the GNOME download page
from the home page.

https://bugzilla.gnome.org/show_bug.cgi?id=666131

Signed-off-by: Andreas Nilsson <anilsson@redhat.com>