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
#8530

Signed-off-by: Erik Lundell <[email protected]>

Author: Erik-Lundell