Docs: 'Stateless RoT Service' application reference

- The implementation details, necessary support in manifest
  tooling, and programming API change for stateless service.
- Application recommendation of developing stateless RoT
  services.

Change-Id: Ibc8e9e68353212d3704b960dda7a2752464cad1e
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>