Update Getting Started documentation

Our "getting started" landing page is not how to get started on Mesos, it's how to build. Build instructions should exist in their own file, and the getting started page should be more like a landing page such as the community page is. As someone who has onboarded other developers, I speak from experience saying we need real "getting started" info.

This work was started at the docathon at Mesosphere a couple weeks ago.