Add a pub use example

[XiaoPengMei]

Closes #1234

Summary

• add a small pub use example to the use declaration chapter
• show how a module can re-export an item through its public interface

Validation

• mdbook build
• mdbook test

Notes

• AI-assisted: I used Sisyphus to help draft and validate this documentation change, and I reviewed the final diff before submission.

[rustbot]

r? @marioidival

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• Owners of files modified in this PR: @marioidival