Add community contribution processor for text editors

What does this MR do and why?

Adds a new processor to notify the Knowledge team when community contributions are submitted for text editors.

The Knowledge team is often unaware of community contributions to text editors until they are already merged. This processor follows the same pattern as AutomatedReviewRequestUx to send Slack notifications for early awareness.

Expected impact & dry-runs

Low volume expected — based on historical data, there are approximately 10-20 community MRs per year with Category:Markdown or Category:Text Editors labels.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations => N/A
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @gitlab-org/plan-stage/knowledge
  • Relevant Slack channels => #knowledge-community-contributions

Checklist

• Slack channel #knowledge-community-contributions created
• Verify processor works after merge