Add Storybook to Primer CSS

[langermank]

Adding a Storybook instance to primer/css for an improved CSS development experience. This PR contains a very quick story example for Button and a brief Getting Started page that I plan to add to. ActionList will have a much more complete story, which is being built in a separate branch.

This is currently for local development only. We'll likely want to configure a deploy in a future PR.

• Fixes: Storybook for Primer CSS #1660

/cc @primer/css-reviewers

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Latest commit: e352f68

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[jonrohan]

Looks good 👍🏻 Pulled it down and tested locally.

Thanks for adding this! Love the theme colors too