Documentation/process: updates to the PGP guide

Small tweaks to the Maintainer PGP guide:

 - Use --quick-addkey command that is compatible between GnuPG-2.2 and
   GnuPG-2.1 (which many people still have)
 - Add a note about the Nitrokey program
 - Warn that some devices can't change the passphrase before there are
   keys on the card (specifically, Nitrokeys)
 - Link to the GnuPG wiki page about gpg-agent forwarding over ssh
 - Tell git to use gpgv2 instead of legacy gpgv when verifying signed
   tags or commits

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