doc: Use west instead of Git to clone Zephyr

Since west is no longer included in the Zephyr repository, instruct
users to install and use west in order to obtain the Zephyr source code.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <torsten.rasmussen@nordicsemi.no>