Site reference docs for new nav layouts and the pagination rune

Acceptance Criteria

site/content/runes/nav.md updates

• Intro rewritten to reflect that nav is the single primitive for sidebar, header menubar, footer columns, and section-landing cards — not just sidebars
• layout attribute added to the Attributes table with values vertical (default), menubar, columns, cards
• collapsible boolean attribute added to the Attributes table
• defaultOpen attribute added to the Attributes table (comma-separated group titles)
• A ## Layouts section with one subsection per value, each containing a {% preview %} block showing the markdoc source and the rendered output:
  • ### Vertical (sidebar) — current behaviour, default
  • ### Menubar (header) — example with two groups (Product, Resources), each with a couple of items; note dropdown / hamburger behaviour
  • ### Columns (footer) — example with three groups (Product, Resources, Legal)
  • ### Cards (section landing) — example with explicit slugs; show enriched output with title + description + icon
• A ## Collapsible sidebars section with a {% preview %} example demonstrating {% nav collapsible %} and explaining the auto-open-current-section behaviour
• A short ## Frontmatter fields used by cards section documenting that the cards layout reads title, description, and (new) icon from each linked page's frontmatter, with a tiny YAML example
• A ## Mobile behaviour callout (or section) noting that menubars collapse to a hamburger below the breakpoint and that columns stack — both handled by Lumina, no author work required
• Cross-references to WORK-184 are NOT included in the rendered docs; the doc page itself is timeless

site/content/runes/pagination.md (new file)

• Frontmatter: title: Pagination and a description line
• Intro paragraph naming the use case (sequential reading — tutorials, ordered docs, recipes) and contrasting with breadcrumb and nav
• ## Basic usage section with a {% preview %} showing {% pagination auto /%} placed in a _layout.md
• ## Explicit prev / next section with a {% preview %} showing {% pagination prev="install" next="configuration" /%}
• ## Ordering section explaining the priority — declared nav order → frontmatter order → directory order — with a short worked example
• ### Attributes table covering auto, prev, next, scope, prev-label, next-label
• ### Common attributes table identical to other rune docs (the shared layout / spacing / inset / tint / bg row)
• A note on boundary behaviour (no wrap-around; first page → no prev link, last → no next)
• A note on the scope="section" value and when to use it

Cross-cutting

• site/content/runes/_layout.md updated to add pagination to the appropriate group in the rune catalog sidebar (likely under Site, next to nav, toc, breadcrumb)
• site/content/runes/rune-catalog.md (or wherever the catalog is generated / curated) lists the pagination rune
• All new {% preview %} blocks render correctly (verify with cd site && npm run dev)
• Authoring docs in site/content/docs/authoring/ are reviewed; if they have a section that lists rune examples or content-model patterns that should mention the new layouts, update them — otherwise no changes needed