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The old search.rst has been dropped as it seems outdated.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