Add note about copying metadata from issue

What does this MR do and why?

This MR adds a note about using /copy-metadata to automatically populate a community MR to be triaged. Maybe we could eventually have the Gitlab Bot prepopulate that as well, it does with GitLab team members' MRs.

Expected impact & dry-runs

This could cut down time needed to triage an MR.

Action items

Handbook update: gitlab-com/www-gitlab-com!104211 (merged)

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review