Epic: Refit Rackula docs into a task-first Starlight product-docs shell

[ggfevans]

What to build

Refit the Rackula documentation site into a task-first Starlight product-docs shell using the approved design spec committed in 4a148d2 and stored at docs/superpowers/specs/2026-07-04-starlight-docs-refit-design.md.

The epic keeps the site on Astro + Starlight while improving the left sidebar IA, homepage launch path, visual clarity, page structure, and Deploy section.

Acceptance criteria

• Left sidebar uses Start, Build layouts, Reference, and Deploy as the primary map.
• Homepage acts as a compact documentation launchpad rather than a broad card catalogue.
• Starlight styling is polished without rebuilding the whole layout system.
• Deploy exposes Docker, LXC, Proxmox Community Scripts, TrueNAS Marketplace, and Unraid as planned/not yet available.
• npm run build passes after the implementation issues land.
• Desktop and mobile navigation are visually reviewed.

Known related issues

• docs: Write self-hosting guide #8 documents Docker self-hosting.
• Document Proxmox LXC and Community Scripts install path #23 documents Proxmox LXC and Community Scripts.
• Document TrueNAS Apps Market install path #24 documents TrueNAS Apps Market.

Blocked by

None - can start immediately.