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

[irvinesunday]

Given the Links annotation in the CSDL:

We would want to add the external doc. link (from the href property) to the generated OpenAPI doc. in the ExternalDocs property of operations:

[baywet]

just a quick note on the openapi example, it contains a MD extension that shouldn't be here.

[irvinesunday]

just a quick note on the openapi example, it contains a MD extension that shouldn't be here.

FYI @millicentachieng to action on API Doctor