Deletes the document with the specified doc_id from the specified external source.

Deletion is asynchronous: the request returns 204 No Content once the deletion is accepted. Follow it with GET /knowledge-management/external-sources/{external_source_id}/documents/{doc_id}/status, which reports DELETE_PENDING, then DELETED, or FAILED with error code DELETE_DOCUMENT_ERROR.
Note: This operation is only valid for external sources of type CUSTOM. Attempts to perform it in other types of external sources are rejected with a 400 Bad Request error.