Add the documentation links from Links annotations in CSDL to OpenAPI Operations

[millicentachieng]

Closes #230

Example:

  /admin/serviceAnnouncement/healthOverviews:
    description: Provides operations to manage the healthOverviews property of the microsoft.graph.serviceAnnouncement entity.
    get:
      tags:
        - admin.serviceAnnouncement.serviceHealth
      summary: Get healthOverviews from admin
      description: 'A collection of service health information for tenant. This property is a contained navigation property, it is nullable and readonly.'
      externalDocs:
        description: Find more info here
        url: https://docs.microsoft.com/graph/api/serviceannouncement-list-healthoverviews?view=graph-rest-1.0
      operationId: admin.serviceAnnouncement.ListHealthOverviews
      parameters:
         #truncated
      responses:
        '200':
          $ref: '#/components/responses/microsoft.graph.serviceHealthCollectionResponse'
        4XX:
          $ref: '#/components/responses/error'
        5XX:
          $ref: '#/components/responses/error'
      x-ms-docs-operation-type: operation

[baywet]

thanks for taking in the changes