Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4122
Thanks for the explanation. I missed the fact that it was present in the installation chapter before. Thus, moving it makes sense to me. However, should we change the `note` directive to a `seealso` directive?