docs(driver): add package docs to mock and sidecar backends

internal/driver/mock/mock.go

@@ -1,3 +1,4 @@
+// Package mock provides an in-memory device driver that records actions for tests.
 package mock
 
 import (

internal/driver/sidecar/client.go

@@ -1,3 +1,4 @@
+// Package sidecar implements the device driver by talking to the native sidecar over gRPC.
 package sidecar
 
 import (