GitLab code review process and documentation updated

The review process documentation has been updated to be coherent with what the robot expect.

The main take away is.

  • +1 means acknowleded by, aka LGTM, aka a quick look into it.
  • +2 means reviewed by, aka deep look, line by line, review
  • +3 means locally tested (and also deep look review)
  • In order to be considered mergeable, any MR needs at least one +2, and +1s do not stack.