Add Related Items slot (#64)

[Cabonie]

Renders a content item's related items as a linked list below the content via a new RelatedItems component registered for the belowContent slot in @plone/layout. Includes EN/DE/IT labels, a how-to guide for hiding or replacing the slot, and tests.

Caution

The Volto Team has suspended its review of new pull requests from first-time contributors until the release of Plone 7, which is preliminarily scheduled for the second quarter of 2026.
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• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I succesfully ran code linting checks on my changes locally.
• I succesfully ran unit tests on my changes locally.
• I succesfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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If your pull request closes an open issue, include the exact text below, immediately followed by the issue number. When your pull request gets merged, then that issue will close automatically.

Closes #64

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📚 Documentation preview 📚: https://plone-aurora--119.org.readthedocs.build/