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://github.com/redpanda-data/documentation-private/issues/
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 configuration to support navigation enhancements and multiple content sources. The primary change expands start_paths in the content source to include data-platform and self-managed alongside home. Two new extensions are added to the Antora pipeline: a conditional home page redirect extension and a unified, config-driven navigation extension. The @redpanda-data/docs-extensions-and-macros dependency is bumped from ^4.0.0 to ^5.0.0 to provide these new extensions, and the extension ordering is adjusted accordingly.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Suggested reviewers

• micheleRP

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description addresses the main change but includes incomplete template sections: the issue number placeholder is not filled in, page previews are missing, review deadline is blank, and no checks are selected.	Fill in the issue number reference, add page preview links if applicable, specify a review deadline, and select appropriate checks from the template.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main changes: bumping docs-extensions-and-macros to v5 and configuring new extensions in the playbook.
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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