Document exemplar usage of the Conan API

It's really good to have a reference for the conan API and what's currently there is useful in pointing in the right direction, I think it would be really valuable to have code snippets that exemplified the usage of each of the API interfaces. It's useful to know where things should be imported from, how to construct the data required as arguments, the types that the interface has and where this can be obtained from. Examples will help to solidify an understanding of how to use it.

As it stands, I'm inspecting the `conan` codebase to confirm my suspicions about usage requirements, which isn't great from a UX perspective at the moment.