Add code-review overview pages

[jmacdotorg]

Add a new overview page-or-pages about how to perform code reviews with CodeRabbit.

This should focus on the workflow of performing automatic code reviews with CodeRabbit. I suspect we'll want at least two new pages out of this: a high-level overview of all the facets of performing reviews with CodeRabbit (including on-demand reviews, other commands, and chat), and a deeper dive into just the topic of automated reviews.

Update other pages as necessary for context and flow.

[linear]

ENG-1253 Add a code-review overview page