They really don't belong in /contributing.

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.

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The old search.rst has been dropped as it seems outdated.

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