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As presented to the TSC, Zephyr's out-of-box experience for new
developers is, well, complicated.  A number of suggestions were
presented including simplifying the getting started material to present
a straight-forward path through the setup and installation steps through
to getting a sample application built, flashed, and running.

This PR is a work-in-progress towards addressing this OOB experience
with a minimal-distractions version of the GSG. Alternatives, warnings,
and material that could lead the developer astray were moved to
alternative/advanced instruction documents (based on the previous
separate Linux/macOS/Windows setup guides) and a new "Beyond the GSG"
document.

We do take advantage of a sphinx-tabs extension for synchronized tabs to
present OS-specific instructions: clicking on one tab will display all
same-named tabs throughout the doc.

We hope (and will continue evaluating) that this new GSG gets developers
set up quickly and then we can send them along to other documents to
continue learning about Zephyr and trying other sample apps.

Thanks for all your previous feedback that I've worked
into this new version.
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>