Add operational runbook documentation

[SaridakisStamatisChristos]

Summary

• add a comprehensive operational guide covering monitoring, alerting, capacity planning, and incident response
• document a repeatable upgrade and rollback playbook for kll_sketch deployments
• surface the new operational documentation from the README for easy discovery

Testing

• not run (documentation-only changes)

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https://chatgpt.com/codex/tasks/task_e_68e61f345f7c83339ccd98ba601981e2

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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