add context to include in the generated data

[pchampin]

Those context files can be used to turn the files in w3c/groups into JSON-LD.

More precisely:

• w3c-groups.json: each object in the array should be extended with "@context": "./contexts/group.jsonld"
• all-repositories.json: each object in the array should be extended with "@context": "./contexts/repository.jsonld"
• {group}/repositories.json / {group}/others.json:
  each object in the array should be extended with "@context": "../../contexts/repository.jsonld"
• {group}/group.json: the object should be extended with "@context": "../../contexts/repository.jsonld"

(all the above assuming that the contexts folder is copied at the root of the w3c/groups repo).