WIP: Improve docs

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.
[ ] Document this structure.
[ ] Incorporate feedback

Things that should be done in a separate branch once changes are merged:
[ ] 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?