Write an explanation and comparison of the different computing environments/runners

[tsibley]

These are a bit better described in the installation docs now, but there's still a lot left unsaid, like why you'd want to use one over the other in different situations, and how the CLI is intended to make them quickly interchangeable. Would also be good to describe how Snakemake is the default workflow executor, but that this is also meant to be interchangeable depending on the build repo.

[tsibley]

Completed in #303. Related to #288.