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

src/docs/userguide/arcanist_extending_lint.diviner

@@ -3,6 +3,14 @@
 
 Explains how to customize existing linters.
 
+This is a configuration guide that helps you set up advanced features. If you're
+just getting started, you don't need to look at this yet. Instead, start with
+the @{article:Arcanist User Guide}.
+
+This guide explains how to refine lint behavior. To configure lint in the first
+place, see @{article:Arcanist User Guide: Configuring a New Project} and
+@{article:Arcanist User Guide: Lint}.
+
 = Overview =
 
 Arcanist ships with a number of linters which you may want to reuse in whole