Changed "by a admin" to "by an admin"

Fix typo ("An" before a word starting with a consonant)

This includes two examples for product attributes.

Also fixes two "document not referenced" warnings.

Fixes #794.

Fixes #1266

Two links needed to be corrected.

It's a great idea, but confusing when empty. Replaced by links to
how_to_customise_models which contains some explanations.

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.