Is your feature request related to a problem? Please describe.
In order to keep the schema DRY we want to extract common objects and reuse that definitions.

Describe the solution you'd like

• create definitions directory for the new files
• extract leaders definition to person.json schema
• extract sponsor definition to sponsor.json schema
• extract repository definition to repository.json schema
• extract other definitions to the separate schema files for further reuse
• create standardized tests for all fields in the new schema (null/undefined/non-unique/invalid)
• keep the current chapter/project schema compatible by referencing local schema files, e.g.

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "sponsor": {
      "$ref": "./schema/definitions/sponsor.json"
    }
  }
}