docs(readme): add explainer video link above demo video

[aaight]

Summary

• Adds the new "What is Cascade?" explainer video (https://youtu.be/gcDLVZw6HS8) to README.md
• Places the explainer video above the existing demo video since it's more introductory ("What is Cascade?" before "Watch the demo")
• Uses the YouTube auto-thumbnail URL (https://img.youtube.com/vi/gcDLVZw6HS8/maxresdefault.jpg) following the same clickable-thumbnail-image markdown pattern as the existing demo video

Test plan

• View rendered README on GitHub — explainer thumbnail displays correctly as a clickable image
• Click explainer thumbnail → opens https://youtu.be/gcDLVZw6HS8
• Click demo thumbnail → still opens https://youtu.be/HMfFtj2i_Mw
• Layout looks clean with both thumbnails stacked vertically

Trello card: https://trello.com/c/JfORoBqV/506-i-have-prepared-an-explainer-video-what-cascade-is-lets-link-it-from-readmemd-https-youtube-gcdlvzw6hs8

🕵️ claude-code · claude-sonnet-4-6 · run details

[nhopeatall]

LGTM — Clean docs-only change. The explainer video link follows the same clickable-thumbnail markdown pattern as the existing demo video, placement order is logical ("What is Cascade?" before "Watch the demo"), and the YouTube video ID is consistent between the thumbnail URL and the link. All CI checks pass.

🕵️ claude-code · claude-opus-4-6 · run details