Add documentation for persistent storage grpc API

Add documentation for persistent storage grpc API

This change adds documentation for the recently merged AGL Persistent
Storage API (Gerrit Change 29996).

Bug-AGL: [SPEC-5187]

Change-Id: Ic9743c95885b2d60b3728f58568a7e27aefe6a59
Signed-off-by: Ludwig Schwiedrzik <ludwig.schwiedrzik@d-fine.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/30013
Reviewed-by: Walt Miner <walt@linux.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>