Document First official CLI (Toolbox)

https://issues.jboss.org/browse/THREESCALE-1362

Comments from Content Plan review:
This component is new, and so may need a new docs section, plus some general description of it. A pre-requisite to using it is installing it and I assume we should document that.
We need to document installing the officially supported RHEL version on RHEL, but also how to get and install the versions for MacOS and Windows directly from the upstream repo.