Updated installation instructions to use the new Symfony Installer

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -
@javiereguiluz couldn't the build process import the conf.py available in the doc repo and then overwrite only the settings it needs to change ? This would avoid you to change the build config for such changes done by the doc team to add new syntax highlightings for instance