Tutorial navigation highlight should update as you scroll

[bvaughn]

When you click on side-nav items, the page scrolls to that section (with 4px blue span in front of the link text). If you scroll manually to those parts, the side-nav doesn't update to show where you're at.

You can see it on a screenshot, An Interactive Component should be active.

Originally reported by @vicodinchik via facebook/react/issues/10984

[jxom]

Hey @bvaughn! Can I tackle this one? :)

[bvaughn]

This issue is all yours! 😄

I've added an "in-progress" label so that others will know not to start work on the issue. If you change your mind about the issue, no worries! Just let me know so that I can remove the label and free it up for someone else to claim.

Cheers!