Add some timeline suggestions to trainee maintainership

1. Historically, most backend trainee maintainers have completed their training in five to seven months. This isn't a target for new trainees, but can serve as a useful signpost.
2. As we removed maintainer buddies, suggest that trainees set up a check-in with someone (either their manager or an existing maintainer) roughly every six weeks, to prevent things from getting stale.

Also make sure the template and the handbook are consistent about who creates the MR (it's the trainee).