Create initial offboarding template and documentation

This change adds a new offboarding process for contributors who want to leave the GitLab community. It creates an automated issue template that guides departing contributors through removing their access to community resources, with optional steps like uninstalling development tools and cleaning up access tokens. The system automatically creates an offboarding issue within 5 minutes when someone requests to leave, providing a structured checklist and support channels to ensure a smooth exit process.