docs: Add doc about WAL PVC sizing

[clayton-cornell]

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Slack discussion: https://raintank-corp.slack.com/archives/CSN5HV0CQ/p1785958163738969

Summary: The Alloy documentation correctly recommends deploying as a StatefulSet with persistent storage for the WAL directory, but provides no guidance on how much storage to provision. This is a common gap that forces users to rely on Prometheus upstream docs or community trial-and-error.

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