Improve Storybook API docs for menu component

[jattasNI]

Pull Request

🤨 Rationale

Continuing the work done in #2027 in service of #824. This PR covers the menu component.

Example of why this is needed: after documenting the nimble-menu-item change event I searched for usages in SLE and found dozens of instances where apps were using the keyboard inaccessible click event instead. I'm fixing them in this PR.

👩‍💻 Implementation

1. Refactored menu MDX and story files to follow patterns from previous PRs. I made some judgement calls about where to document child components vs examples; open to feedback on how it turned out!
2. Added more shared documentation strings and used them in several existing stories.

🧪 Testing

Manual storybook inspection

✅ Checklist

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