Original PR:symfony/symfony#26648

An issue was already opened some times ago:#9641
and closed by2099d48 but there is actually no mention to theValidationMiddleware and how to use it (ValidationFailedException should also be explained, as it allows getting the violations).

Only theValidationConfiguration is mentioned inhttps://symfony.com/doc/current/components/messenger.html#adding-metadata-to-messages-envelopes but I think this middleware deserves its own section/guide.