Updated installation instructions to use the new Symfony Installer

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -
I still remember the "varnish tabs" nightmare and the "best practice tab" horror story. I'm afraid that I'm too old to still believe that anything related to software is easy or it will work at first.