Arm backend: update docs to new template (#9712)

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
https://github.com/pytorch/executorch/issues/8530

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