doc: west: add project groups

This is a new feature that allows you to associate projects with
groups, then force west to ignore a subset of the projects list by
disabling project groups. This can be selectively overridden via
configuration file to make west stop ignoring the projects within an
individual workspace.

An example use case is to define a project whose Git repository
requires credentials to pull. Putting such a project in a group named
"foo" and disabling group "foo" would make "west update" ignore the
project unless the user specifically configures the workspace to allow
"foo". After that, "west update" will pull the project as usual.

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