coreos-download: restyling coreos download page
Restyles the Fedora CoreOS Download page based on the initial proposed
design https://pagure.io/fedora-websites/issue/964#comment-576026.
Moreover, dustymabe and abai thought it could be a good idea to add release
stream info and release notes to the top level coreos page:
https://getfedora.org/en/coreos/.
Signed-off-by: Allen Bai <abai@redhat.com>