This expands the introductory docs for the User Guide for Review Board,

providing a high-level summary of Review Board and some notable

integrations and features, before proceeding to linking to other

sections of the documentation.
This can help teach people more about the benefits of the product

without jumping through all the pages first, or backtracking to other

parts of the site.
Some of the links have been renamed as well, helping keep the links on

this page in sentence casing. One link, "Reviewing diffs," has been

changed to "Reviewing code (diffs)", to help clarify that page.

Built and viewed the docs. Checked for bad links, spelling errors, and

build errors.