now that we install it from an external repository.

Refactored the previous best-practices document as an ADR.
Previous document: https://github.com/edx/edx-platform/blob/07f588517b11dea238aadff80c9daf3b36e1a3aa/docs/best_practices.rst

This is part of a larger effort to split packages out into independent,
composable, PyPI installable Python packages.

While this package itself isn't monumental, it represents the bottom of
the dependecy tree and can be split out before any other packages.

This basically commits the transpiled CoffeeScript JS (with minor
cleanup) and removes coffee build support.

A tiny amount of support for xblocks exists, because external users
may have xblocks with coffee. But no coffee in our tree anyway.

Covers some of the basics of how our Sass is compiled in edx-platform.

- Remove old docs
- Move testing.rst
- Update READMEs and index.rst for docs.

This makes it possible to refer to multiple book from a single app.

This also means that Open edX branches will be able to pass bok-choy
tests (at least if the books have already been built.)