Add the 'Community contribution' label to community merged MRs

When creating or changing a new triage policy or triage package:

• Consider adding documentation to the handbook pages for Triage Operations: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/handbook/engineering/quality/triage-operations/index.html.md
• Consider announcing to the appropriate Slack channels or week-in-review

Closes #424 (closed).

Closes https://gitlab.com/gitlab-core-team/general/-/issues/63