[Messenger] Added a learn more section

[Nyholm]

There should be a link from the component page to the How to use Messenger page

[Nyholm]

Thank you

[javiereguiluz]

@Nyholm in all the old (2.8) components we added this warning at the top of the article to end the confusion of readers thinking that the "component article" is the article explaining how to use the component inside a Symfony app. So we can do the same here ... and add your Learn More section too. Thanks!

[javiereguiluz]

Update: in 3bbd822 I've removed the "Learn More" section because there are still no articles about Messenger in the messenger/ directory and the doc build was failing. We'll re-add it once we add new independent articles about Messenger. Thanks!

[Nyholm]

It should still be a reference to /messenger though.

https://github.com/symfony/symfony-docs/blob/master/messenger.rst

[javiereguiluz]

Yes ... but we now add the reference to /messenger.rst at the beginning of the article, like we do in all the other component articles. When we add more independent articles about the Messenger, we can add the "Learn More" section.

Keep in mind the comments of the other PRs too: if the new proposed articles are too small, they should be just sections of the main article. We're working hard to simplify Symfony Docs, so we no longer merge very short articles. Thanks for your understanding!

[Nyholm]

Good!

Yes, thank you.