west: require v0.6.0 or higher

The main change is the elimination of the bootstrapper, a design flaw
/ misfeature.

Update the documentation to be compatible with the 0.6.x releases as
well. This has to be done atomically, as there were incompatible
changes.  Make use of the versionchanged and versionadded directives
to begin keeping track of how these APIs are evolving.

(Note that west 0.6.0 will remain compatible with the extension
commands in Zephyr v1.14 LTS as long as that is still alive. This
change is targeted towards Zephyr 2.0 users.)

This requires a bump in the shippable container and allows us to
simplify the west_commands test procedure.

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