Right nav should scroll

[lai-bianca]

The scroll bar on the right nav doesn't auto-scroll.

Current behavior:
When you are scrolling through the main body of content, the relevant section is highlighted in the right nav, which is great. But once you scroll past the last section in the right nav, the nav does not scroll to match the page.

Expected behavior:
When you are scrolling through the main body of content, the right nav should also auto-scroll so that the section you're looking at is also visible in the right nav.

Attaching a video that shows current behavior:

right-nav-scroll.mp4