Documentation: PGP: update for newer HW devices

Newer devices like Yubikey 5 and Nitrokey Pro 2 have added support for
NISTP's implementation of ECC cryptography, so update the guide
accordingly and add a note on when to use nistp256 and when to use
ed25519 for generating S keys.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>