docs: add StatefulSet PromQL queries

[yasicar]

What this PR does / why we need it:

• Documents practical StatefulSet monitoring guidance by adding a new docs/queries/statefulset-monitoring.md cookbook.
• Links the main docs index to those PromQL examples so operators can find rollout, availability, and alerting patterns quickly.

How does this change affect the cardinality of KSM: does not change cardinality (documentation-only update).
Which issue(s) this PR fixes: none.
Fixes #: N/A

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