docs: clarify security implications of space quotas

[pigeio]

What is this change?

This PR adds a specific "Security Note" to the Space Quota documentation in the Maintenance Guide. It clarifies that quotas are not just for resource management, but serve as a defense mechanism against Availability-based Denial of Service (DoS).

Why is this change necessary?

While investigating the write path in server/etcdserver/api/v3rpc/quota.go, I observed that the check() function runs prior to the main Put logic.

Many operators view quotas purely as a storage administrative task. Highlighting that this protects the cluster from entering a "read-only" panic state (due to disk exhaustion) emphasizes the security value of this configuration.

How was this tested?

• Verified code logic in etcd/server/etcdserver/api/v3rpc/quota.go.
• Local Hugo build to ensure the alert renders correctly in the documentation.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pigeio
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[k8s-ci-robot]

Hi @pigeio. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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