Add single-sourced connected client monitoring docs

[kbatuigas]

Description

Related PR: redpanda-data/docs#1470 - this must be merged first.
Related PR (Data Plane API): redpanda-data/api-docs#30 - Data Plane API reference links will not work until this is merged

Resolves https://github.com/redpanda-data/documentation-private/issues/
Review deadline: 19 Nov

Page previews

What's New in Cloud
Manage Throughput > View connected client details
Monitor Redpanda Cloud > Throughput > Identify high-throughput clients
Configure client connections
rpk cluster connections list

Checks

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

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📝 Walkthrough

Walkthrough

This pull request adds documentation for a new feature: connected client monitoring in Redpanda Cloud. The changes include updating the Antora playbook to pull a specific documentation branch (DOC-1780-cloud-client-monitoring), reorganizing client connection configuration pages with aliases for backward compatibility, creating two new reference pages for rpk cluster connections commands, adding a "Connected client monitoring" subsection to the what's new guide, and updating several include directives to use versioned references (25.3@).

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

• Multiple documentation page additions following a consistent pattern (rpk-cluster-connections.adoc, rpk-cluster-connections-list.adoc, configure-client-connections.adoc)
• Simple include directive version updates throughout (25.3@ qualifier additions)
• Single-line branch change in Antora playbook configuration
• Straightforward page reorganization with alias for backward compatibility

Possibly related PRs

• BYOVPC support for secrets and Iceberg REST catalogs  #313: Modifies local-antora-playbook.yml to change which documentation branches are pulled for documentation sources
• DOC-1673 single source client connections in cloud-docs #412: Modifies the "Configure Client Connections" / configure-availability content reorganization with include-based pages
• DOC-1682 Monitoring not supported on Serverless #417: Modifies modules/manage/pages/monitor-cloud.adoc include path versioning and monitoring documentation content

Suggested reviewers

• paulohtb6
• micheleRP
• Feediver1

Pre-merge checks and finishing touches

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	❓ Inconclusive	The PR description includes related PRs, page previews, and a review deadline, but the issue number placeholder is unfilled and no checklist items are marked.	Fill in the GitHub issue number and mark at least one checkbox to indicate the type of change (New feature, Content gap, Support Follow-up, or Small fix).

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The PR title 'Add single-sourced connected client monitoring docs' directly aligns with the main change: introducing documentation for connected client monitoring features using single-sourced content.
Linked Issues check	✅ Passed	The PR addresses DOC-1780 by adding comprehensive documentation for connected client monitoring features including new rpk command docs and whats-new content.
Out of Scope Changes check	✅ Passed	All changes are in-scope: adding/reorganizing connected client monitoring docs, restructuring related configuration pages, and updating include directives for versioned content.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[Feediver1]

lgtm