doc: west: troubleshoot no west on PATH on windows

Depending on how people set up their Python, west might not be on
PATH.

I'd like to have a special-purpose step by step guide for how to
handle this in the documentation to save support time when this
happens. I think it's worth special casing since west is the first
runnable program that pip installs onto a new user's system when
they're following the GSG.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>