Update links.yml to run checks on https://ultralytics.com

[glenn-jocher]

copilot:all

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update to the link checker in CI and README badge link.

📊 Key Changes

• Expanded the exclusion list in the link checker (lychee) to include tiktok.com and fonts.googleapis.com.
• Added workflow steps to download and run broken link checks on the Ultralytics website for the main branch.
• Updated the README badge link for the broken link check status to remove the branch specifier.

🎯 Purpose & Impact

• Enhanced Accuracy: The expansion of the exclusion list will reduce false positives during link checks, improving CI accuracy.
• Additional Quality Check: By downloading and checking the Ultralytics website, the quality control extends to ensure external content linked from Ultralytics is reliable.
• Improved Transparency: Updating the README badge provides a direct view of the link check status from the main page, enhancing visibility for contributors and users.