Improve OpenAPI document

[schrieveslaach]

As noted in #243 the OpenAPI documentation provides invalid response description (see here:

paths:
  /apps/:
    get:
      summary: List deployed review apps.
      responses:
        '200':
          description: ''
          content:
            application/json:
              schema:
                type: object
                properties:
                  "^[a-zA-Z0-9_-]":
                    # TODO: This is an array and the documentation seems to be wrong all the time…
                    $ref: '#/components/schemas/Service'

This should have been an array of services.

#243 and #257 extended the API with content-negotiation based API versioning and the new versions should be documented too.