doc: west: add v0.6.1 documentation

West v0.6.1 has been released. The main feature is that "west update"
now avoids communicating with the network by default when project
revisions have already been fetched.

This allows for offline operation and significantly speeds up the
command (it's 10 times faster on my machine and home network).

There are other miscellaneous changes as well; document them also.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>