Add doc-rooted site example version, update guidance

[chalin]

• Contributes to Reintroduce support for doc-rooted sites #2504
• Fixes Pages missing in the section index of docs-only sites #2499
• Fixes Top-level section landing pages are always rooted in a docs-only site #2492

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• Doc-rooted site variant: add config/doc-rooted/permalinks.yaml using Hugo permalinks to publish docs at site root (/)
• Docs rewrite: replace legacy "docs-only" cascade/type guidance with concise doc-rooted permalinks instructions; add migration note and caution about URL conflicts
• Version menu enhancements: add section headers (**Versions**, **Variant**), kind: latest / kind: home support, markdownify for bold names, pagelinks: false option, separator validation
• SCSS: extract td-navbar-dropdown-item-icon mixin; add dropdown-item-home (house icon) and dropdown-item-latest (semi-bold + suffix) styles
• Build scripts: make Hugo environment configurable via env var; add _hugo-dev-test for template metrics; move --logLevel info to base _hugo script
• Config restructure: move hugo.yaml → config/_default/hugo.yaml; inline sites YAML anchor; remove unused archived_version param