Add docs for managing separate Helm deployments for user code

Summary:
This decoupling of Dagster infrastructure and User Code deployments
has been available for while - here, we add formal documentation on
how to achieve this decoupling.

Test Plan: make dev

Reviewers: johann

Reviewed By: johann

Subscribers: catherinewu

Differential Revision: https://dagster.phacility.com/D8075