coreos: add steps for verifying each artifact
    
    Add steps for verifying the signature and checksum of each
    downloadable artifact, after expending the
    `Verify signature & SHA256 buttons`. This makes the
    verification steps simpler as the commands can be copied
    and entered as shown.
    
    Includes generating a URL-encoded CHECKSUM file for each
    artifact, which users can then run `sha256 -c` on.
    
    Also drop `coreos-download.css`, as a dedicated
    class from there is no longer needed.
    
    Signed-off-by: Robert Fairley <rfairley@redhat.com>