pages/yubikey: add pam_u2f alternative
    
    Apparently there is also pam_u2f as alternative to pam_yubico which is a
    more general approach using the open FIDO2 / U2F standard. It's much
    easier to setup as there are fewer possibilities and decisions to make.
    
    This approach also supports more security keys like SoloKey v2 and
    NitroKey 2/3 which could open possibility to create a dedicated security
    key page for general key support in Fedora Linux.