doc: Add Nimtable observability tool reference to README

[yingjunwu]

Added a note about Nimtable as an observability tool for Iceberg tables.

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What's changed and what's your intention?

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

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Release note

[Copilot]

Pull Request Overview

This PR adds a promotional reference to Nimtable, an Iceberg table observability tool, in the README under the Iceberg lakehouse section.

• Adds a single line mentioning Nimtable with a link to its GitHub repository