Update docs directory structure

I'd like to give the docs a bit of an overhaul. This is a work-in-progress and feedback is most welcome!

Things that need to be done before merging:
- [x] Switch to a directory structure closely following the proven model that Django uses, simplified for our needs.
- [x] Get rid of old _draft folder, keeping any files worth keeping.
- [x] Update main `index.rst` to reflect changes.
- [x] [Document](https://github.com/tangentlabs/django-oscar/blob/feature/betterdocs/CONTRIBUTING.rst#writing-docs) this structure.
- [ ] Incorporate feedback

Things that should be done in a separate branch:
- [ ] Merge old `conventions.rst` with `contributing.rst`
- [ ] Merge old `my_model_customisations_arent_[...].rst` into existing docs.
- [ ] Merge old `offers.rst` with existing offers documentation
- [ ] See if old `search.rst` is still valid.
- [ ] `getting_started` and `take_a_peek` - overlap?
- [ ] Split up contributing guidelines in overview, tests, vagrant, conventions and writing docs.

Related issues: https://github.com/tangentlabs/django-oscar/issues/567