feat!: Use semantic HTML <nav> element to wrap table of contents

[kaushalmodi]

There's a small chance that the user might need to tweak their site CSS for this change.

• [ ] Do not default to nav element. The TOC looks weird based on how some CSS libraries render the nav tags (see below). It's possible that that's the issue with that CSS library. But it would be good to not break rendering on any sites by leaving the TOC default element to the currently used div. Instead may be add a way to customize the TOC element.

---

nav elements with nested ul/li elements rendered by mvp.css