docs: replace callout blockquotes and bold NOTE markers with alert

[AR21SM]

Fixes #1084.
Audited all etcd documentation versions (v2.3–v3.7) and replaced attention-grabbing callouts that used NOTE:, NOTE:, and > blockquote syntax with {{% alert %}} .

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @AR21SM. Thanks for your PR.

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[wendy-ha18]

/ok-to-test