Rewritten from scratch the chapter about installing Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4122, #4089
@javiereguiluz IMO, the right way to use the installer would be ``symfony new 2.3`` rather than ``symfony new 2.3.23``, because there is no reason to create a new project based on an outdated patch version. So as soon as the installer supports such command and the doc is updated for it, nobody will be affected by older versions not being installable (and nobody should have a legitimate reason to be affected today anyway)