doc: west: 0.7.0 documentation

Reflect changes in the APIs made since 0.6.0. These will also need to
be added to the release notes.

Some automatic directives weren't generating the desired output, so
either do it by hand or let new 0.7.0 docstrings supply the information.

Try to better group the content into sections.

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