docs: reduce README banner image size

[noobakong]

Summary

• Replace the README banner from Replace PageIndex banner image #436 with an optimized JPEG.
• Preserve the existing 1471 × 491 dimensions and visual appearance.
• Reduce the served image from 857 KB to 203 KB, approximately 76% smaller.

Testing

• Confirmed the uploaded asset is a 1471 × 491 JPEG.
• Verified the banner URL and surrounding HTML are unchanged apart from the asset ID.
• Not run (documentation image-only change).