doc: west has moved to allowlist/blocklist

The west manifest import feature now uses the terms "allowlist" and
"blocklist" instead of "whitelist" and "blacklist", respectively.

The old terms are still supported by the tool for compatibility, but
the docs for this feature are going to use the new terms exclusively.

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