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>