The wiki is moving! Developer and admin docs are in the process of being moved to mkdocs. Other documentation should be moved to their respective Gitlab wiki spaces on lab.civicrm.org.
2017-06-09: There is an ongoing problem with logins from civicrm.org ("network error"). This may take some time to be resolved, if it gets resolved. In the mean time, you can request an account in the infrastructure channel on CiviCRM's chat platform.