Put one document into the set, leaving the rest of its membership alone.

A pair with the DELETE below, and neither is PUT /{id}. That route replaces the whole set — name, description, guidance, documents and live pages — which is right for the form where somebody is composing the lot by hand and wrong for a chip somebody clicked: a one-click remove that resent the whole set would overwrite whatever another tab had changed since the page loaded, and would delete and re-insert every website row to no purpose. _add_members already exists for exactly this reason on the reading path.

Idempotent. A document already in the set keeps the role it has, for the reason _add_members gives: a collision here is a race with another edit, and the first edit's answer is the safer one to keep.