Docs: Contributing guidelines and legacy docs

- [x] Merge old `conventions.rst` with `contributing.rst`
- [x] Merge old `my_model_customisations_arent_[...].rst` into existing docs.
- [ ] Merge old `offers.rst` with existing offers documentation
- [x] See if old `search.rst` is still valid.
- [x] `getting_started` and `take_a_peek` - overlap?
- [x] Split up contributing guidelines