Updated installation instructions to use the new Symfony Installer

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -
I think we should still say "Installing Symfony", that's the task you have to do in this section. "Symfony Installer" is no longer a task, making the section more of a wiki than a step by step tutorial.