Enable prettier for storybook package

[jattasNI]

Pull Request

🤨 Rationale

Fixes #2136

👩‍💻 Implementation

1. Follow patterns in other packages for .prettierignore, prettierrc
2. Add *.mdx as not ignored file type
3. Run prettier and bask in the glory of consistent formatting

I'm also setting metadata on rich-text-mention-users-guidance.mdx to put it in the Patterns section since I noticed it's appearing publicly.

🧪 Testing

Spot checked the rendered storybook and some of the changed files to see if anything was being egregiously broken.

✅ Checklist

• [x] I have updated the project documentation to reflect my changes or determined no changes are needed.