Scroll progress for the right-side page summary/headings

Implement page position tracking (see e.g. Bootstrap [scrollspy](https://getbootstrap.com/docs/5.3/components/scrollspy/)). Highlight the current heading (e.g. with bold) in the tree and show a progress line above the tree. See [here](https://www.linode.com/docs/guides/getting-started/) for an example.