Document the new Symfony installer

[javiereguiluz]

Symfony installer is undergoing a major overhaul for the Symfony 2.6 version, as detailed in https://github.com/symfony/symfony-standard/issues/689

This issue is a reminder that those changes will impact in Getting Started tutorial and the Official Book.

[javiereguiluz]

I'll take care of this issue.

[stof]

Actually, it should be documented for all versions of the doc, given that the installer supports Symfony 2.3 too

[javiereguiluz]

@stof. You are right. This issue is more complex than it looks like, because we have to document this in the download page, the Quick Tour tutorial, the book and the best practices book. That's why I have a plan to avoid repetitions.

[wouterj]

@javiereguiluz I think the best option is to wait until we have just one document (we should decide on the strategy very soon). It seems like updating this takes lots of time, while it would be all removed pretty quickly.

[javiereguiluz]

@wouterj I'm afraid that I don't fully agree on your last proposal. The chapter about installing Symfony is absolutely critical. That's why I propose #4654 to quickly fix all the current problems. Once it gets merged we can plan the best strategy to consolidate the four different installation docs (the book, the quick tour, the best practices book and the download page).

[wouterj]

I think this one is fixed now, @javiereguiluz ?

[javiereguiluz]

@wouterj yes, this is fixed now and all the remaining issues are tackled by this meta-issue: #4672