Development documentation

This starts a devel-docs/ subdirectory, with the start of a Sphinx book for the development documentation. I intend to move things like COMPILING.md and CONTRIBUTING.md here, as well as the architecture and design documents.

There's a chapter on setting up a development environment, which should be useful for Outreachy and Summer of Code interns - and contributors in general.

Blatantly stolen from the Firefox Source Tree Documentation and the Guide to Rustc Development.