Add responsibility section detailing DRIs for broken master

Why is this change being made?

The DRI and SLO is not clear for ~"master:broken" issues. This is to clarify the responsibility of the Engineering Productivity team and the counterpart team that the issue relates to.

Summary of changes

• Move master pipeline failure notification from #development to #master-broken channel
• Clarify the triage and resolution phases of resolving ~"master:broken"
  • Engineering Productivity team is the DRI for triage - https://clarify-dri-broken-master.about.gitlab-review.app/handbook/engineering/workflow/index.html#triage-broken-master
  • SLO for triage (master pipeline failure to issue created and assigned to DRI) is 12 hours
  • Merge request author is DRI (merge request author’s team is backup) for resolution - https://clarify-dri-broken-master.about.gitlab-review.app/handbook/engineering/workflow/index.html#resolution-of-broken-master
  • SLO for resolution (assignment of issue to issue closure) is 12 hours

Actions

These changes will go into effect on 2020-01-09. To support this Merge Request we need to:

• Communicate final form in Engineering Week in Review, Company Call, #development, #backend, #frontend - Kyle W week of 2020-01-06
• Merge this MR
• Update the Slack integration for broken pipelines to #master-broken

Closes #5812 (closed) Closes https://gitlab.com/gitlab-org/quality/team-tasks/issues/278

Quicklink to review handbook page: https://clarify-dri-broken-master.about.gitlab-review.app/handbook/engineering/workflow/index.html#broken-master