As decided in T258953, we will represent the documentation of ZObjects not in JSON, but in a second MCR slot. This task is for implementing that system.
The wikitext is stored in the second MCR slot, for all languages. The ZObject handler looks the slot up, gets the appropriate doc from the slot, and displays it between the Header and the actual ZObject rendering.
We also need to implement how to handle editing, history, watchlist, etc.