Docs improvement: Quick start guide

The idea behind this improvement lays down on my previous experience with Antora. I tried to install Antora several times with no luck: there were troubles with Node version (I was forced to downgrade to v.8.x), nodegit, empty local repos, etc.

From the documentation, it is easy to understand how to get Antora work with demo data and demo playbook, but when you are trying to start from the ground up, things get a bit harder.

My idea is to rework the existing sections and combine them into a quick start guide. For example, Run Antora section can be divided into two parts: the first one related to running Antora with demo data, the second one with your own data. In the second part, it will be easier to provide users with a sample module, so that they can just download/unpack it to the source folder and launch Antora locally. In this way, users will get the basics and after that will be ready to dive into details with more confidence.

I can contribute these changes on my own or in collaboration with @graphitefriction after discussing the structure and details.