chore: bump docs-extensions-and-macros to v5 and wire new extensions

[kbatuigas]

Add conditional-home-redirect and unified-navigation extensions, and extend docs-site start_paths with data-platform and self-managed.

Description

Resolves https://redpandadata.atlassian.net/browse/
Review deadline:

Page previews

Checks

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

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📝 Walkthrough

Walkthrough

This PR updates the Antora documentation build pipeline with an extended scope and new processing extensions. The dependency @redpanda-data/docs-extensions-and-macros is bumped to version 5.0.0. The Antora playbook configuration is then updated to include two additional documentation sections (data-platform and self-managed alongside the existing home), and two new extensions are configured: a conditional home-page redirect extension and a unified navigation extension that handles configuration-driven and page-level navigation attributes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: bumping the docs-extensions-and-macros dependency to v5 and configuring the new extensions in the playbook.
Description check	✅ Passed	The description follows the template structure with a clear summary, Jira ticket placeholder, page previews section, and checks checklist. The Jira ticket link is not filled in but the template placeholder is present.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/docs-extensions-v5

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