Our Review Board manuals have started out with just a plain Table of

Contents, with no introduction or real flow to the rest of the

documentation. This could be frustrating for new users.
This change is the first in a series that aims to modernize our

documentation, guiding users through getting going and pointing them to

more useful content without having to dig or search.
The new front page contains actual content, guiding the user through the

main pieces of the product, and grouping these pieces into digestable

sections.
The content on the pages linked from the main page haven't changed (with

the exception of some pages gaining an :orphan: tag to prevent Sphinx

errors when building). Edits will probably be coming to some of that

pages as I go.

Built the docs and went through them. Didn't see any new errors.