Split up contributing guidelines

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

my_model_customisations_arent_picked_up now lives in a subsection of the
recipe how_to_customise_models.

The old conventions.rst is now merged into the new coding-style.rst.

The old search.rst has been dropped as it seems outdated.
Docs: Centralise sandbox instructions

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.
Docs: Add debugging trick for model customisations

Was posted on the mailing list.
Docs: Added documentation for tox tests
Docs: Merge tests/README.rst into docs structure
Docs: Move design decisions and dev environment

They really don't belong in /contributing.
A few minor corrections to the docs
Include snippet for better documenting of model fields

See http://djangosnippets.org/snippets/2533/
Add simple address app docs (including autodoc)
Merge pull request #580 into releases/0.5
Add missing settings to docs

Plus some general reorganisation.
Fix a few sphinx warnings
Rework the docs index page