Polish the README header, features and table of contents

[devmaxxx]

Summary

• Centered header with tagline, npm version, downloads, types, CI and license badges, and a quick-links line.
• Features list rewritten with bold leads, including that parsing uses the browser's own HTML5 parser.
• Table of contents, a note under Install that browser entries need a DOM (links the server-rendering recipe), changelog and releases links in Contributing, and the copyright holder under License.

Test plan

• prettier --check README.md
• Every badge URL returns 200
• Rendered README on GitHub looks right