feat: add public pose datasets index with license-safe corpora (Fixes #20)

[laurentketterle-hub]

Summary

Public pose datasets index for PoseGuide — docs/DATASETS.md with 10 license-safe corpora.

Closes #20

Deliverables

• docs/DATASETS.md — 10 public pose/photography datasets with license, format, and URL info
• License categories (CC-BY, BSD, Research-Only)
• Integration notes for PoseGuide compatibility
• tests/test_datasets_index.py — fixture adapter test (5 test cases)

Datasets Included

1. MPII Human Pose, 2. COCO Keypoints, 3. LSP, 4. LSP Extended, 5. MPII-TRB,
2. FreiHAND, 7. Panoptic Studio, 8. Human3.6M, 9. 3DPW, 10. AIST Dance DB

[laurentketterle-hub]

/attempt 20

[laurentketterle-hub]

Friendly ping 👋 — this PR has been open for 3 days. Is there anything needed for review? Thanks!