R1:055e8f07ab56

Improve arcanist documentation

Summary:
There's some good feedback from Wikimedia here:

http://lists.wikimedia.org/pipermail/wikitech-l/2012-August/062252.html

Try to improve on some of it. In particular:

- Make it clear that /arcanist/ is not where you should be (D3235).
- Provide better connections from "Arcanist User Guide" to other documents.
- Provide a "Quick Start" guide with a simpler set of instructions that links to richer documentation.
- Reorganize the project setup guide to put…