doc:it_IT: translation for maintainer-pgp-guide

It translates the maintainer-pgp-guide in Italian.

Signed-off-by: Alessia Mantegazza <amantegazza@vaga.pv.it>
Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>