Arm backend: update docs to new template

The old doc follows the tutorial template and has thus been moved
to a new file tutorial-arm-ethos-u.md. backend-arm-ethos-u now
instead follows the new backend-template.md.

Additinally, make sure both files follow the points iterated in
pytorch#8530

Signed-off-by: Erik Lundell <erik.lundell@arm.com>
Change-Id: If77ba6c54d9c726a369f98c28cf42712cd32d511

Author: Erik-Lundell