This adds a small docs page covering the new Document Review settings

page in the admin UI sidebar. It briefly discusses the feature and

broker URL settings, and includes information on the broker status, with

a screenshot.
It's the first page to include an admonition stating that this is a

feature in Review Board Plus and Enterprise. We'll want to standardize

on language and appearance for these notices going forward.

Built the docs (along with a temporary intersphinx rule pointing to

Power Pack, which is in the User Roles change). Tested that links worked.
Tested the screenshots.
Checked for spelling issues.