Encode devops stages parent/child structure in either categories.yml or stages.yml

With the process of building our dashboards and label migration. We should store the single source of truth on devops stage parent-child structure in the handbook project.

Context:

• gitlab-org/quality/triage-ops#172 (closed)
• https://quality-dashboard.gitlap.com/groups/gitlab-org/stages

Comments from the dev metrics meeting.

Jason: Idea is to encode this in data in the handbook. => Lyle: isn’t it categories.yml and stages.yml. => Jason: There is no label field. We should extend it.