coreos: add initial boiler plate code for release notes
    
    The design discussion of Fedora CoreOS release notes is located at https://github.com/coreos/fedora-coreos-tracker/issues/194. This commit adds boiler plate code for future addition of Fedora CoreOS release notes to the website, though currently there's no metadata available and no actual rendering will happen on the website.
    
    Signed-off-by: Allen Bai <abai@redhat.com>