Reorganise the main documentation page

[craigfowler]

I'd like to improve the docs homepage. I want to make it more accessible to beginners, and to link some of the newest content. I'd like to organise the content into these three categories on the home page.

Content for beginners

• Quick start (a "Hello world" Screenplay project)
• Basic concepts

Content for intermediate users

• Advanced concepts
• Using Screenplay in normal circumstances

Concepts for advanced users

• API reference
• Extending Screenplay