docs: update sample dataset download link

[Lovlace777]

Summary

• Replaces the stale CleanVision sample dataset download URL with the accessible Hugging Face dataset URL.
• Updates the README quickstart and the tutorial notebook so they point to the same working zip file.
• Fixes the same tutorial download command from wget - nc to wget -nc while touching that example.

Validation

• curl.exe -I -L --max-time 30 https://huggingface.co/datasets/Cleanlab/cleanvision-sample-images/resolve/main/image_files.zip -> HTTP 200, Content-Type: application/zip, Content-Length: 82341589.
• python -m json.tool docs/source/tutorials/tutorial.ipynb -> passed.
• git diff --check -> passed.
• Full Sphinx docs build was not completed locally: the base Python environment does not have Sphinx, and a temporary docs environment setup timed out before Sphinx was installed.

Notes

• Fixes Broken link #274.
• Documentation-only change; no runtime compatibility impact expected.
• No follow-up work expected.

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