Add Slack notification on links CI failure

[glenn-jocher]

copilot:all

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Implementation of automated Slack notifications for failed link checks in documentation.

📊 Key Changes

• Added a new workflow step to notify via Slack when a link check fails.
• The notification triggers only for the 'ultralytics/docs' repository.
• It activates on a scheduled run or a push event.

🎯 Purpose & Impact

• Enhanced Monitoring: Automatically notifies the team when documentation link issues arise.
• Improved Responsiveness: Helps maintain up-to-date docs by promptly addressing broken links.
• Streamlined Workflow: Saves time by catching errors without manual monitoring, enabling quick fixes. 🛠️

(Note: While the diff provided only shows the addition of new lines for Slack notification, the summary assumes that the previous parts of the file handle link checking in the documentation website - context inferred from the workflow job names and commands.)