Docs restructure

This pull request organises the docs into sections
 - Getting started - Tutorial for beginners
 - Core components - Documentation for the main part of wagtail (Split up into topics like Pages, Images, Snippets, etc)
 - Contrib components - Documentation for contrib apps
 - How to - Tutorials (like deployment, performance, etc)
 - Reference - Docs which provide an overview of all the commands, template tags, hooks and settings Wagtail has
 - Releases - Release notes