croissant: add rai:hasSyntheticData, absolute HF contentUrls, JSONL column extracts (all 4 NeurIPS checker checks pass)

docs: switch Repository link to anonymous.4open.science

Update README.md: fix URLs and task_categories

Update croissant.json: fix URLs and task_categories

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Upload LICENSE

Upload croissant.json

Upload benchmark/realworld_test.jsonl

Upload benchmark/mixed_items.jsonl

Upload benchmark/base_items.jsonl

initial commit