croissant: add top-level schema.org citation

croissant.json: unprefix RAI fields, add @context aliases (NeurIPS RAI compatibility) (#8)

croissant.json: split aggregated recordSets to one per fileset (source cardinality fix) (#7)

Update Croissant 1.1 metadata: fix forbidden chars, paths, recordSet IDs (#6)

Add Croissant 1.1 metadata (croissant.json) (#4)