Added root openapi.yaml file to reference the managedAttribute and

[xuemeigan]

metadata yaml as well as generated html documentation files :
#51

For now we can only have one linked external doc due to below open PR:
OAI/OpenAPI-Specification#1034

[cgendreau]

The PR should not include html files.

[xuemeigan]

Currently the path can link to the actual spec file. To see the individual spec in document format, I think we need the external doc link, which will point to the html file.

[cgendreau]

Paths should be included using the proper $ref:

paths:
  /v1/metadata: 
    $ref: 'metadata.yaml#/paths/~1v1~1metadata'
  /v1/managed-attribute:
    $ref: 'managedAttribute.yaml#/paths/~1v1~1managed-attribute'  

But we need 1 path element per $ref. It would also be ok to simply have 1 document named object-store-api.yaml

[cgendreau]

The id of the acMetadataCreator should not point to an agent, it's the identifier only.