Add external_docs parameters to get_openapi method

The OpenAPI class is being instantiated without the externalDocs parameter.

fastapi/openapi/utils.py

@@ -489,6 +489,7 @@ def get_openapi(
     contact: Optional[Dict[str, Union[str, Any]]] = None,
     license_info: Optional[Dict[str, Union[str, Any]]] = None,
     separate_input_output_schemas: bool = True,
+    external_docs: Optional[Dict[str, Any]] = None,
 ) -> Dict[str, Any]:
     info: Dict[str, Any] = {"title": title, "version": version}
     if summary:
@@ -566,4 +567,6 @@ def get_openapi(
         output["webhooks"] = webhook_paths
     if tags:
         output["tags"] = tags
+    if external_docs:
+        output["externalDocs"] = external_docs
     return jsonable_encoder(OpenAPI(**output), by_alias=True, exclude_none=True)  # type: ignore