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>