feat: add docref to capability statement

[carvantes]

Add the operations registered in OperationDefinitionRegistry to the capability statement

I chose to add a documentation field on the implementation of the operation instead of just copying the documentation from the OperationDefinition from the IG since docs for operations are sometimes vague and it is expected that servers document the particularities of their implementation. e.g. our docref documents that it does not generate documents on demand.

The fragment of the cap statement for docref looks like this:

{
  "type": "DocumentReference",
  ...
  "supportedProfile": [
    "http://hl7.org/fhir/us/core/StructureDefinition/us-core-documentreference"
  ],
  "operation": [
    {
      "name": "docref",
      "definition": "http://hl7.org/fhir/us/core/OperationDefinition/docref",
      "documentation": "This operation is used to return all the references to documents related to a patient..."
    }
  ]
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Bingjiling]

LGTM