The howto recipe on shipping has been rewritten to describe the new
changes to shipping functionality. The old shipping app reference has
been merged in too to avoid duplication.

- Expand the ref/apps docs. (Still lots to do here.)
- Split up the index page links for clarity.

They were missing the autodoc documentation for views (abstract) and
abstract models.
Also added two docstrings to the views in the promotions app to have
them showing in the autodoc.

To set a base for upcoming docs changes, the docs directory structure
has been streamlined to follow a simplified version of Django's docs.

The old _draft folder has been removed. Both files from there and the
current docs that don't fit nicely into the new structure are in _old/
for the moment.