Idea: Storybook specimens

I’ve been wondering lately if Catalog is able to leverage stories written for storybook as part of writing documentation. Storybook has a rich ecosystem of addons and integrations (https://github.com/oblador/loki) that can improve test and development workflows greatly.

Proposed usage:

```storybook
kind: Forms
story: Validations
```

It will be important for the Storybook specimen to be able to pull the code for a story and render it alongside the demo as with the React specimen today.

[jerstucki]

Cool!

It’s already possible to add custom Specimens. It’s just not documented yet 😅

The simplest form of it is this:

<Catalog specimens={{storybook: StoryBookSpecimen}} />

The YAML/JSON config from the Specimen inside the Markdown maps directly to a Specimen’s props.

There’s a Specimen higher-order component to provide layout and theme info to custom Specimens. You may not need to use that to prototype something.

What’s not really possible yet is to hook into the Catalog CLI’s build system. If you need to do that, let me know what would be useful to add there.

[jerstucki]

Closing in favor of #423