feature(www): Scroll-sync on docs table of contents

[tesseralis]

Summary

• When scrolling through a doc page, the active item should switch to the current section (much like the tutorial sidebar
• When scrolling through a doc page, the hash at the top should update to the current section

Basic example

1. Go to docs quick-start
2. Click on one of the table of contents links
3. Scroll around the page

The "active item" is the item originally selected in the hash, not the currently focused element, possibly causing confusion.

Motivation

Much like how the tutorials are scroll-synced to ease navigation, adding a scroll sync would make it easier to navigate around the docs. We have a lot of docs that have long sections, and it's easy to lose one's position.

Implementation

Most likely, we can adapt the scroll sync sidebar into a useActiveHash hook. Instead of passing in an item list, we can just query the DOM for page headings (or use the headings query to get them).

[jlkiri]

I'd like to try this one.

[jlkiri]

Added in #21762