Update the maintainer PGP guide

This series updates the guide to match terminology used in the upstream
"protecting code integrity" guide [1] and brings the documentation in
line with the latest developments in the GnuPG world:

- uses "Certify key" instead of "master key" terms to remove common
  confusion that the "Certify key" is somehow able to restore lost
  private subkeys
- removes keyserver instructions because keyservers have largely gone
  semi-extinct due to GDPR enforcement and just general neglect
- adds a link to the kernel.org PGP keyring documentation
- updates information about ECC curve support among the devices the
  guide talks about (Yubikeys are able to use ED25519 curves with the
  latest firmware updates)
- adds a section on using PGP-signed patches with b4 and patatt

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>