Migrate the guide site from Hugo to Astro + Starlight

[dcmcand]

Summary

Replatforms the "Building a Software Pack" guide (packs.nebari.dev/building-a-software-pack/) from Hugo to Astro + Starlight, themed by @nebari/starlight@^0.1.6. Maintainers keep writing Markdown; the shared theme handles presentation, and the guide now matches the portal and other packs.

What changed

• Astro + Starlight at docs/site/ (Bun), replacing the Hugo project. All 7 content pages migrated (+++ TOML -> --- YAML, _index.md -> index.md); bodies unchanged. Files stay .md so the Helm {{ }} code blocks are preserved.
• Dynamic base: /building-a-software-pack/ on main (the portal Worker strips the prefix), / on PR previews (served at the pages.dev alias root). Files emit at dist/ root.
• Base-safe links: a small rehype plugin prefixes internal root-absolute Markdown links/images with the base (Astro does not do this for body content).
• Sidebar: two groups, with Concepts added to "Getting Started".
• Branding + search + edit links: Nebari theme (magenta accent, Poppins, logo linking to the portal, footer), Starlight Pagefind, and GitHub "Edit page" links.
• CI: docs.yml swapped from Hugo/Go to Bun/Astro; computes BASE_PATH per environment, deploys docs/site/dist, and comments the preview URL. Preserves the fork-PR deploy skip and Cloudflare project.

Testing

bun test in docs/site: 11 pass (355 assertions) covering pages render + titles, sidebar structure/order, base-safe link resolution, branding CSS + portal logo link, Pagefind bundle, edit-link shape, and the rehype plugin unit tests.

This PR's preview deploy exercises the CI path; production serves on merge to main.

[github-actions]

📄 Docs preview for feat/astro-starlight-docs:
https://feat-astro-starlight-docs.nebari-software-pack-template.pages.dev