Document Go client APIs

[sipsma]

Right now both the LLB and gateway client libraries are essentially undocumented, with new users having to look at examples (in the Buildkit repo and elsewhere) or just the underlying implementation to know how to use them.

I think at a bare minimum it would be good to add docs in the code for these interfaces. More examples and/or guides would be good too, especially for the LLB client library.

Also, something like "integrator docs" that explains what the terms LLB, gateway, frontend, etc. would be good too to help those who are trying to create higher-level tools using Buildkit get started in the first place.