Page TOC should support auto-scrolling

[chalin]

As the user browses through a page, the TOC is updated to show which section is being viewed (by highlighting the corresponding TOC entry).

The problem is that for TOCs that don't fit in the view (e.g. Flutter for React Native developers), the TOC itself isn't scrolled to keep the active heading in view.

[parlough]

I played around with implementing this, but it ended up not resulting in a great user experience. Many other common doc site tools don't implement this behavior and such long pages aren't too common, so I'm going to close this for now.

I'm implementing a new TOC design for wide and narrow layouts in #11701 that should hopefully improve visibility into the current section. After that, if anyone has further concerns about getting lost in the TOC, please let open a new issue with more details. Thank you!