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.

It's such a core concept, it deserves a dedicated document. Moved some
of the "why" from the how-to into it as well.

They really don't belong in /contributing.

They were getting quite long. They've been split up in several
subsections. Some of them are borrowing heavily from the Django docs.

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.