doc: west: Overhaul documentation to match current model

Overhaul the west documentation so that it matches the model that is
currently implemented in the west repository.
This model is no longer subject to change in the short or mid term,
since it has been selected as the only viable one for multi-repo
management using west.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>