Add Cosmos3 Edge Diffusers cookbook

[atharvajoshi10]

Summary

• add Cosmos3-Edge text-to-image, text-to-video, and image-to-video sections to the Diffusers audiovisual cookbook
• use the Edge 480p, 121-frame, no-audio sampling configuration
• document the Edge quickstart settings in the audiovisual README

Validation

• generated a 832x480 text-to-image result with Cosmos3-Edge
• generated 121-frame 832x480 text-to-video and image-to-video results with Cosmos3-Edge
• validated notebook JSON, payload construction, Python cell syntax, and git diff whitespace