docs(tab): add selected icon example

[pollychen-lab]

Summary

Add a viewable code example to the Tab documentation page.

Changes

• Add an interactive example showing icon and selectedIcon.
• Add matching block metadata so the docsite generator registers the example under Tab.
• Use only existing Tab and TabList APIs.

Validation

• pnpm build
• Docsite data generation
• Docsite test suite: 206 tests passed
• Docsite TypeScript check
• ESLint
• Prettier
• Repository pre-commit checks

Fixes #2765

[vercel]

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[meta-cla]

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