various fixes
- remove references to the former qlf runners
- fix formatting issues (the markdown syntax seems to differ from gitlab's flavour)
- mention that the runners are using the docker executor
Prochaines maintenances programmées: lundi 03/06, lundi 01/07, lundi 05/08
Pour plus d'informations: https://doc-si.inria.fr/display/SU/Gitlab