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

[dgrisonnet]

/triage accepted
/assign @CatherineF-dev

[dgrisonnet]

@yasicar could you please take a look at the CI failures?

[yasicar]

/triage accepted

[yasicar]

@dgrisonnet @CatherineF-dev I have fixed the CI failures.

[mrueg]

/lgtm

Thanks for your contribution @yasicar

[k8s-ci-robot]

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