Added a reference link of usecase fork_app management command.

I found some docs changes in my git stash that seemed useful. We removed
support for support when removing Django 1.6 support, so we can remove
mention of South.

A custom app needs to inherit from Oscar's app config. This ensures the
correct app label, and that any startup code (receivers, etc.) is
executed.
Per the camp ground rule, the fork_app command has been slightly
refactored.

The recently introduced management command makes customising easier, and
the docs have been updated accordingly. The how-tos could still do with
updating to the new command, but at least the main docs are current.