service - api docs relationship

We want api docs to be able to belong to a service.

• api docs allows for service selection in edit so customers with free floating api docs can attach them to a service
• api docs menu item in service submenu
• if imported as part of "discovering a service" from the cluster, we should be able to also update that api doc when the service is updated ("rediscovered")
• Expose API/Service on api_specs liquid drop as requested in this Github issue