minor #15027 [Mailer] doc(mailer): add Sendinblue to transports supporting tags and metadata (pierreboissinot)

This PR was merged into the 5.2 branch.

Discussion
----------

[Mailer] doc(mailer): add Sendinblue to transports supporting tags and metadata

<!--

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `5.x` for features of unreleased versions).

-->

Commits
-------

1859757 doc(mailer): add Sendinblue to transports supporting tags and metadata

Author: OskarStark

mailer.rst

@@ -1209,6 +1209,7 @@ The following transports currently support tags and metadata:
 * Postmark
 * Mailgun
 * MailChimp
+* Sendinblue
 
 Development & Debugging
 -----------------------