Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4122
I'm not sure if that's necessary. Why would one like to install, for example, `2.3.20` instead of the most recent `2.3.23` version (and when it's possible to use `symfony new 2.3` this won't be less than a problem at all, right?)?