Add docs-only labeller

What does this MR do and why?

gitlab-org/technical-writing#631 (closed)

This MR introduces an automation to label or unlabel merge requests with docs-only changes.

Expected impact & dry-runs

Run locally:

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (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