feat: made homepage a real landing page

[JoshuaKGoldberg]

PR Checklist

• [x] Addresses an existing open issue: fixes #93
• [x] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

When	Screenshot
Before
After

Also adds the O'Reilly link redundantly to the list of books in the header. I'd been going back and forth on whether I want it there a second time, since it's already the primary link.