[Mailer] Mention the `SentMessageEvent` and `FailedMessageEvent`

[wkania]

The https://github.com/symfony/symfony/pull/47080 PR was created to solve the problem of debugging and to get information about email after it was sent. The section about debugging does not mention those events. Added some links for better navigation.

For example what debug returns:

< 220 ESMTP SYMFONY.COM
> EHLO [127.0.0.1]
< 250-smtp.symfony.com
< 250-PIPELINING
< 250-SIZE 157286400
< 250-AUTH PLAIN LOGIN PLAIN LOGIN PLAIN LOGIN
< 250-AUTH=PLAIN LOGIN PLAIN LOGIN PLAIN LOGIN
< 250-ENHANCEDSTATUSCODES
< 250-8BITMIME
< 250 SMTPUTF8
> AUTH LOGIN
< 334 VXNlcm5hbWU6
> ZXhhbXBsZUBzeW1mb255LmNvbQ==
< 334 UGFzc3dvcmQ6
> U3ltcGhvbnkg.aXMgQXdlc29tZQ==
< 235 2.7.0 Authentication successful
> MAIL FROM:<[email protected]>
< 250 2.1.0 Ok
> RCPT TO:<[email protected]>
< 250 2.1.5 Ok
> DATA
< 354 End data with <CR><LF>.<CR><LF>
> .
< 250 OK. ID: a20fb6ebbc54d22b

P.S. I see that the checks now can find repeated words:

mailer.rst ✘
 1765: The word "the" is used more times in a row.
   ->  ``FailedMessageEvent`` allows acting on the the initial message in case of a failure and some