Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4122
I agree that it doesn't make sense. The problem right now is that 95% of Symfony versions cannot be installed with the installer. That's why I think it's important to vaguely say *"some past versions cannot be installed"*.