minor #21604 [Mailer] Mention MJML as an alternative to Inky (javiereguiluz)

This PR was merged into the 6.4 branch.

Discussion
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[Mailer] Mention MJML as an alternative to Inky

This is an alternative to #21585. Inky is still the Symfony recommendation, so we cannot remove it yet. But, let's mention MJML as a valid alternative.

Commits
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897a3f3 [Mailer] Mention MJML as an alternative to Inky

Author: javiereguiluz

mailer.rst

@@ -1163,6 +1163,11 @@ the entire email contents from Markdown to HTML:
 Inky Email Templating Language
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. tip::
+
+    Symfony recommends Inky, but you can also use `MJML`_, a more actively
+    maintained alternative for responsive email templates.
+
 Creating beautifully designed emails that work on every email client is so
 complex that there are HTML/CSS frameworks dedicated to that. One of the most
 popular frameworks is called `Inky`_. It defines a syntax based on some HTML-like
@@ -2012,3 +2017,4 @@ the :class:`Symfony\\Bundle\\FrameworkBundle\\Test\\MailerAssertionsTrait`::
 .. _`S/MIME`: https://en.wikipedia.org/wiki/S/MIME
 .. _`Scaleway`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Mailer/Bridge/Scaleway/README.md
 .. _`SendGrid`: https://github.com/symfony/symfony/blob/{version}/src/Symfony/Component/Mailer/Bridge/Sendgrid/README.md
+.. _`MJML`: https://github.com/mjmlio/mjml