[26.2] Improved cluster health summary

[kbatuigas]

Description

This pull request updates the Redpanda release notes to document new features and configuration options, most notably the addition of cluster health metrics export as Prometheus metrics. It also removes several older release note sections and details on new configuration properties, streamlining the release notes and focusing on the most impactful changes.

New Feature Documentation:

• Added a section on cluster health metrics, describing how Redpanda can now export a cluster health summary as Prometheus metrics for alerting on cluster health issues (e.g., brokers down, leaderless partitions, missing controller) without polling rpk cluster health. The metrics mirror rpk cluster health output and are disabled by default.
• Provided instructions for enabling cluster health metrics by setting the health_monitor_metrics_enabled property and restarting brokers, with a reference to the documentation for available metrics and query examples.

Release Notes Streamlining:

• Removed detailed sections and descriptions for multiple features, configuration properties, and changes to default values, focusing the release notes on the most significant new feature (cluster health metrics).

Resolves https://redpandadata.atlassian.net/browse/DOC-2016
Review deadline: 23 July

Page previews

• What's New in Redpanda (updated)
• Public Metrics (updated)
• rpk cluster health (updated)
• Monitor Redpanda: Cluster health (updated) — where the new Cluster health section renders

Preview links resolve once the Netlify build for this PR completes.

Checks

• New feature
• Content gap
• Support Follow-up
• Small fix (typos, links, copyedits, etc)

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[travisdowns]

Looks good, thanks Kat!

[mattschumpert]

Surprised this is opt in @tyson-redpanda . Not high cardinality at all and quite useful in the general case

[mattschumpert]

Surprised nodes in recovery node would not impact cluster health @pgellert @tyson-redpanda

[Feediver1]

Minor, non-blocking suggestions

1. Table grouping isn't visually distinguished: the prose says there are two groups of metrics (freshness vs. overview), but the table lists all 12 rows flat with no visual separation. A subheading row or brief bolding for the two freshness metrics (metadata_age_seconds, refreshes_total) would make the earlier claim easier to verify against the table.
2. Slight redundancy: "By default, each broker refreshes the metrics every 100 seconds. This interval is 10 times the value of health_monitor_max_metadata_age." — both sentences say the same thing two ways; could tighten to one sentence if you want, though it's fine as reviewer-requested clarity.
3. The unrelated start-up → startup typo fix is harmless but is a drive-by change outside the PR's stated scope — worth flagging only because "Small fix" wasn't checked in the PR checklist (not a real issue, just noting it slipped in).

No blocking issues from my read — the technical content (PromQL, property name, defaults) is internally consistent, prior review feedback is fully incorporated, and it already has two approvals. This looks mergeable as-is.