Outline component for to make large articles easier to navigate.

[dyc3]

Long articles are hard to navigate.

We should provide an outline for each article on the left side of the screen. It should be automatically generated based on all of the header tags in the article (h1, h2, etc.). It should highlight what part of the article you are scrolled to, and allow you to click on a title to scroll to it.

This component should be as isolated as possible to make it easier to reuse for non-article pages.