Docs for URL,Dashboard,NavigationBar-Hooks

Review Request #3617 — Created Dec. 3, 2012 and submitted — Latest diff uploaded

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Review Board
master

Reviewers

Extensions documentation for URLHook, DashboardHook, 
and NavigationBarHook.
Ran `make html` in reviewboard/docs/codebase/

No error output from sphinx.

Rendered HTML pass visual inspection.