[docs] Document usdk as an import()able module

[avaer]

usdk can be imported as a module to get access to the cli functionality. The commands are already well-factored and the CLI is just a wrapper around this so the functionality should be on par with the CLI. In fact we already use this in the agent-deployer so we don't have to shell out.

usdk as a module could be useful to people if they want to programmatically access usdk functionality.

Not very high priority.