Improve model docstrings

Only done for the address, catalogue and partner app.

- Added one or two missing docstrings
- Turned some comments into docstrings (prepend #: instead of #)
- Improved upon a few docstrings
Docs: Stubs for catalogue and partner app

- Added stubs for both apps
- Switched to using :automodule: for abstract models instead of
  individual :autoclass:
- Added instructions on creating a Partner and ProductClass instance
  when setting up a new Oscar instance
Docs: Add a glossary

Commonly used terms should be explained there.