Adds rootHeirarchies to the storybook config

Allows us to section off the stories into groups to match https://design.gitlab.com/

Instead of just passing the name of the component to documentedStoriesOf we can pass a hierarchy like this documentedStoriesOf('regions|empty-state', readme). Any stories without a set group will live in the root.

It also configures all the exisiting stories to use the newer format.

Closes #119 (closed)