Add an example based on json schema

[andrea-gioia]

Add an example for v1.0.0 in which the tables field of Schema Object is defined using a Standard Definition Object.

{
  "specification": "json-schema",
  "specificationVersion": "2020-12",
  "definition": {
    "mediaType": "application/schema+json",
    "$ref": "foodmart-schema.json"
  }   
  "externalDocs": "https://json-schema.org/draft/2020-12/schema"
} 

It would be ideal if the schema matched the one used in the existing example, but defined using JSON Schema.