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 more important things earlier on.
• Make it clear that you should commit .arcconfig.
• Provide more hints for initial setup.
• Describe and organize advanced configuration/extension documentation as more clearly separate from basic setup/install documentation.

Test Plan: Generated, read docs.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3236