Document the On Demand depth model

[lemire]

The On Demand front-end depends crucially on how it tracks the current depth. An improper depth state can break the parsing. I think that before we release 1.0, we should have a clearly documented model of how the depth is meant to change.

Furthermore, we should also clearly document the depth and positions at different stages where the user is handed back control.

I got started with a wiki at https://github.com/simdjson/simdjson/wiki/OnDemand-logic