Save a document template

Free, and does not use your document allowance. Storing a layout once is the point: a large layout re-sent on every call is usually the single biggest avoidable cost in a request. Requires X-API-Key -- a stored template needs an owner, and a key is the only identity that exists before a purchase. Placeholders are extracted at save time by the same code that substitutes them, so the declared schema cannot disagree with the template.