UI: Improve docs for compound exports

[mirka]

What?

Follow up to #78184

Documents the expected metadata for @wordpress/ui compound component exports.

Why?

The conventions around Object.assign, displayName, JSDoc placement, and Storybook subcomponents metadata are easy to miss. Capturing them in the package contributing guide should help keep Storybook docs, React DevTools, and IDE tooltips consistent.

How?

Adds guidance to packages/ui/CONTRIBUTING.md for:

• adding primary component JSDoc to both the implementation export and public Object.assign wrapper;
• setting subcomponent displayName values to their full public compound names;
• listing public subcomponents in Storybook metadata with dot-notation keys.

Testing Instructions

Instructions should be comprehensive, and should makes sense.

[github-actions]

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

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[aduth]

Thanks for adding this 👍