Restructure documentation

[frangio]

As part of a bigger review of the documentation (OpenZeppelin/docs.openzeppelin.com#47), we're restructuring the content. The idea is to organize it in a conceptual hierarchy rather than divide it in "Basics" and "Advanced".

The target structure is:

• Overview
• Usage (or perhaps this should be an entry guide?)
• Available Kits
  • Starter / Base
  • Tutorial
  • GSN
• How to create your own