Documentation: update from MVP to demo

Create a dedicated west.yml file for the Zephyr application in order to
minimize the amount of external repositories to be cloned during west
initialiation.

Issue-Id: SCM-4488
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
Change-Id: I86a389abc131f76d44c3e82c0d1921616e91f5ea