[Mailer][Mime] Document PGP/MIME signing and encryption

[lacatoire]

Documents the PGP/MIME support added in symfony/symfony#64540.

mailer.rst gets a new "PGP/MIME Signing and Encryption" section: requirements (the gpg binary and the Process component, no OpenSSL extension), the warning that only the body is protected while Subject, From and To stay in cleartext, standalone usage of PgpSigner/PgpEncrypter, the global pgp_signer/pgp_encrypter configuration, the X-Pgp-Sign/X-Pgp-Encrypt headers that select the messages to protect (sign-then-encrypt, after templates are rendered and before logging), the three on_missing_key modes with the envelope.recipients caveat, and encrypt_for_sender/hide_recipients.

reference/configuration/framework.rst gets the full reference for both configuration nodes.

Fixes #22726

[Spomky]

Thanks for documenting this, as the author of the feature I am happy to see it covered so thoroughly.

One inaccuracy worth fixing:

Unlike S/MIME, PGP/MIME is not applied to every message

That is true of the S/MIME signer, but not of the S/MIME encrypter: SmimeEncryptedMessageListener has always required the X-SMime-Encrypt header too, it just was not documented. Something like "Unlike the S/MIME signer, PGP/MIME is not applied to every message" would avoid the confusion.

For the record, I opened #22764 to document the S/MIME side (#22719), which includes that X-SMime-Encrypt header, the new certificates, on_missing_certificate and encrypt_for_sender options and the listener priorities. It touches the same part of mailer.rst and framework.rst, so whichever gets merged first, the other will need a rebase. Happy to do it on my side.