doc: west: add information about private repositories

How to manage repositories that require authentication to fetch from
using west is a frequently asked question.

TL;DR on the answer is that Git already has credential storage built
in, so our expectation is that there is nothing west needs to do in
particular, as users can select whatever credential helper works for
them, or set up a custom one using something like the GIT_ASKPASS
environment variable.

That's not a very helpful expectation if people aren't aware that
credential helpers exist, though, so let's document that and provide
examples for common use cases, as well as west-specific
troubleshooting advice.

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