[Documentation] code loading/organization docs

[kleinschmidt]

Internally we've been using a "package-based" code loading strategy: work functions/constants are defined in a MyJob package, where you have a name="MyJob" in the Project.toml and a src/MyJob.jl, along with a driver script that does

using Ray
Ray.@ray_import begin
    using MyJob
    using AnotherDependency
end

MyJob.do_work_with_ray()

We shoudl document this pattern and why it's used (i.e., limitations with the existing code loading system, global refs, etc.)