docs: define official pack docs site requirements and gate GA on a compliant site

[dcmcand]

Two related changes:

1. New docs/pack-docs-requirements.md: defines what a compliant official pack docs site is. Stack (Astro + the shared @nebari/starlight theme, source in the pack repo, base: /<slug>/), the baseline two-section layout modeled on the LLM Serving Pack site, and the publication mechanics (Cloudflare Pages project named after the repo, docs_site: true in pack-metadata.yaml, listing in the dashboard's tracked-packs.yaml, which routes packs.nebari.dev/<slug>/). Ends with a compliance checklist.

2. Maturity model update: the release readiness checklist's Documentation section gains a GA row requiring a compliant docs site published at packs.nebari.dev/<slug>/.

Reference implementation for the requirements: https://packs.nebari.dev/llm-serving-pack/. The routing mechanics were verified against generate_routes.py in software-pack-dashboard.