Update docs for the new Welcome Page

[javiereguiluz]

We're working on adding back a "Welcome Page" for new Symfony apps. See symfony/symfony-standard#834

We should show and mention this "Welcome Page" in the Quick Tour, the installation chapter and any other appropriate page.

[javiereguiluz]

I'll take care of this issue myself :)

[javiereguiluz]

This can be closed as fixed by #5628

[wouterj]

You are able to close these issues as well, @javiereguiluz. Don't worry about closing obviously fixed issues like this, you're part of the deciders team :)

[stof]

@wouterj he is not flagged as repo collaborator. Are you sure he has the access to close it ? (well, this one yes as he is the author)