zkvm: clearer API distinction and documentation

[vickiniu]

In #218, the program calls prove_program_predicate, which should generally only be called indirectly when verifying a Predicate::Program variant. Looking at the zkvm codebase right now, it's a little confusing / unclear what functions should be used for building transactions (& which are internal to zkvm), especially since we expose many of them publicly.

Would be great to flesh out the ZkVM API doc with more traditional documentation of the functions, and also only expose as pub functions that should be used in tx-building.

[oleganza]

Yeah, i guess we can expand the Predicates section of the API guide on how to construct and interact with them (and add examples with Program::build/PredicateTree API)