Restructure docs

I think the left-hand nav for Oscar's docs could be improved considerably. At the moment, it just stacks up the various TOCs from `index.rst`. We can improve this by using a `:hidden:` TOC to hand-pick the right links and just use normal lists of links for the current TOCs.

Not quite sure on what the right nav structure should be. Maybe something like:

- Intro (description of what Oscar is, design decisions)
- Getting started (oscar sample sites, installing locally)
- Next steps (eg key questions, customisation, recipes, topics)
- Deploying, upgrading etc
- Support 
- Reference (settings, signals, template tags, management commands)
- Extensions
- Contributing
- Releases