docs: cover both persistent backends in sessions-and-checkpoints

The Storage table and Package Structure were git-branch-only and the package listing had drifted (temporary.go/committed.go no longer exist).

Add a git-refs row to the Storage table, note the store is pluggable with a pointer to the ref-backend doc, and refresh the checkpoint/ package listing to the current files (open.go, registry.go, routing_store.go, fanout.go, refs_store.go, refs_naming.go, pushqueue.go, persistent*.go, ephemeral.go, fsstore/).