Add YouTube Tutorial Link to Collaboration Documentation

[Blaec-CoCalc]

Add YouTube Tutorial Link to Collaboration Documentation

Is your feature request related to a problem? Please describe.
Collaboration features in CoCalc documentation lack a direct video tutorial reference, potentially making it harder for users to get started without visual guidance.

Describe the solution you'd like
Include the YouTube link (https://www.youtube.com/watch?v=jW9xHdL-fUg&themeRefresh=1) at the top of the collaboration documentation page (https://doc.cocalc.com/share.html) to guide new users effectively.

Describe alternatives you've considered
Considered leaving documentation as text-only, but video content can significantly enhance user understanding and engagement.

Additional context
Adding the video link at the top will emphasize its importance and potentially improve the onboarding experience for new users.