Durable local storage proposal

This is a proposal for #598


As discussed, I think this table conflates concepts - specific Volume kinds (HostDir, EmptyDir), policies (Debug), concepts (PackageDir), and lifetimes (CachedDir).

I think, as a doc, we'd be better off talking about the specific kinds of volumes (Empty, Host) first.  Then introduce policies (Debug).  Then maybe talk about the idea of packages and things we'd like to see as new kinds of Volumes.  Then talk about lifetimes and the CachedDir, CachedDirSpec, etc API.

I would leave Remote out for now.