List deduction guides on the record page

[alandefreitas]

Problem

Deduction guides render under their namespace because we follow the AST layout. Documentation conventions expect them on the record page.

Proposed solution

• Re-map the page parent of deduction guides to the associated record.
• Evaluate whether the context should remain the namespace or switch to the record; document the decision.
• Adjust navigation and breadcrumbs accordingly.

Requirements

• Deduction guides appear on the record page in rendered output.
• Navigation remains correct even for nested namespaces.
• Tests confirm relocation for multiple templates and friend cases.