Add documentation on dynamic class loading

It's such a core concept, it deserves a dedicated document. Moved some
of the "why" from the how-to into it as well.
Docs: Remove how-tos on customising mixins and overriding core class

These days, to override a mixin it is only required to override the
OrderPlacementMixin; overriding the app and view is not necessary. It is
just a generic case of overriding a core class. A dedicated how-to is
thus unnecessary.

As overriding a core class is the general goal of any advanced
customisation, it has been merged into the customisation section, which
has been given an overhaul as well.

The section regarding modifying apps will be re-added as part of a
higher-level "the Oscar application tree" kind of document.
Extend docs on application instances and customising them

Understanding the application tree these days is only necessary when
wanting to change URLs or views. Hence, it's been moved from the
customising doc and reworked into the how-to. This reduced some
considerable duplication and hopefully makes the customisation section
clearer.