doc: initial attempt to document setup of pgp support for email. Used an AI assistant to help write this. Basic gpg commands seem to work, but I have not tested this totally. Docs basically follow the setup used for pgp testing in the test suite. It looks like roundup accepts signed emails as well as encrypted and signed emails. But it does not generate signed emails. Also it looks like there is no PGP support for alternate email addresses. Only primary addresses can do PGP emails.