WIP: Improve docs

This is a work-in-progress branch to track improvements to the docs.

- [ ] 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?
- [x] Split up contributing guidelines

Related issues:
- https://github.com/tangentlabs/django-oscar/issues/574
- https://github.com/tangentlabs/django-oscar/issues/567
- https://github.com/tangentlabs/django-oscar/issues/542
- [order status and line status](https://groups.google.com/forum/?fromgroups=#!topic/django-oscar/RILldxUBj2g)