scripts: west: Add documentation for the multi-repo commands

Explain multi-repo concepts like manifests and projects, give an
overview of the currently implemented commands, and give an example of a
potential workflow.

There's no way to submit a multi-repo change for review yet, so this is
still experimental.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
3 files changed