Changes to GET /clinic-config/document-layout — which optional blocks the clinic's invoice, receipt and treatment-plan PDFs carry (ITE-1334), defaults resolved. 404 while the `DocumentLayout` flag is off.

added the optional property letterhead to the response with the 200 status (media type: text/plain)

added the optional property letterhead to the response with the 200 status (media type: application/json)

added the optional property letterhead to the response with the 200 status (media type: text/json)

endpoint added