Add quality maintainer issue template and update handbook

Why is this change being made?

• We have listed guidelines for how to become a quality maintainer !39402 (merged). I'm adding an issue template for whoever wants to start the trainee process.

• Also updated handbook page to reflect the trainee quality maintainer template

ref: https://gitlab.com/gitlab-org/quality/team-tasks/-/merge_requests/37

/cc @ddavison @sliaquat @mlapierre for review 🙏