investigate hydration directory and document related guaranteed/invariates

Currently Skaffold V2 has the concept of a "hydration-directory" that can be assumed to exist across stages or when deleting. There is currently no requirements on when this "hydration-directory" should exist or how generally it has been implemented across current renderers & deployers. This issue tracks detailing the current status of the hydration-directory and outlining what invariates exist and how it should work going forward