[Messenger] Document the validation middleware #10474
Description
Original PR: symfony/symfony#26648
An issue was already opened some times ago: #9641
and closed by 2099d48 but there is actually no mention to the ValidationMiddleware and how to use it (ValidationFailedException should also be explained, as it allows getting the violations).
Only the ValidationConfiguration is mentioned in https://symfony.com/doc/current/components/messenger.html#adding-metadata-to-messages-envelopes but I think this middleware deserves its own section/guide.