Add a separate documentation directory for TEE subsystem since it is a
standalone subsystem which already offers devices consumed by multiple
different subsystem drivers.

Split overall TEE subsystem documentation modularly where:
- The userspace API has been moved to Documentation/userspace-api/tee.rst.
- The driver API has been moved to Documentation/driver-api/tee.rst.
- The first module covers the overview of TEE subsystem.
- The further modules are dedicated to different TEE implementations like:
  - OP-TEE
  - AMD-TEE
  - and so on for future TEE implementation support.
Acked-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231128072352.866859-1-sumit.garg@linaro.org