Document resolver system

Now that the resolver system is largely implemented let's add some documentation around it. The topics to be covered for this issue are:

[ ] The basics of resolvers as a mechanism for customising API results. This should include resolving top-level functions, resolving each returned model/type, and resolving fields within a model.
[ ] The automatically generated resolvers for each model.
[ ] How to override resolvers.
[ ] Resolver chains, and method cut-points.
[ ] How to add links to chains.

furious-luke / polecat