[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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