Improve Getting Started experience

The purpose of this MR is to make the "Getting started" more visible and make it a focus page. This is e.g. done by adding a "Getting Started" button to the homepage and top-nav, so that this page becomes a central hub for all other pages. The idea behind this is the assumption that developers today are used to these kind of "Getting started" pages which helps to get started quickly. All other pages e.g. about OSGi theory are secondary in a sense that they are interesting for people who have managed the first stage (getting started).