feature: support custom document #17
Description
As I can see, until now, custom document doesn't support yet. It's very powerful feature for me who would like to customize layout of the page. It can be possible that there are many layout template made by users. Concept is inspired from next.js. For example of no header / sidebar,
import {Head, Style, Body, Sidebar, Page, Footer} from 'flybook/layout'
export default () => (
<html>
<Head>
<Title title="my boo">
<Style>
<Link src="xxx.css>
</Style>
</Head>
<Body>
<!-- NO SIDEBAR, NO HEADER -->
<Page>
<NextScript />
</Body>
</html>
)