Add Base Astro Site with Nebari Starlight Docs (Experimental)

[jbouder]

Summary

Add a base Astro site using the Starlight docs framework to this repo under a new docs/ directory, following the established pattern from other software packs (e.g. nebari-chat-pack, mlflow-pack, harbor-pack). This is an experimental addition — the initial site will contain only placeholder "coming soon" content, establishing the scaffolding for future documentation of the pack.

Motivation

• Nearly all other software packs already ship a docs/ Starlight site; this pack should match that convention
• The shared @nebari/starlight theme plugin provides Nebari branding (colors, fonts, logo, favicon, footer) for free — no custom styling work needed
• Starlight provides search, sidebar navigation, and dark mode out of the box, so docs authors only write markdown
• Getting the scaffolding merged early lets docs content land incrementally in follow-up PRs

Acceptance Criteria

• New docs/ directory containing a base Astro project with the @astrojs/starlight integration, mirroring the structure used in other packs (astro.config.mjs, src/content/, public/)
• Nebari identity applied via the @nebari/starlight theme plugin with nebari({ logoHref: 'https://packs.nebari.dev/' }), matching sibling packs
• astro.config.mjs respects the BASE and SITE env vars for portal deployment, as the other packs do
• Landing page clearly states the docs are experimental and content is coming soon
• npm install && npm run dev serves the site locally without errors
• npm run build produces a static build without errors
• README.md updated with a short note pointing to the docs/ directory and how to run it locally

Out of Scope

• Writing actual documentation content (follow-up issues once the scaffold lands)
• Deployment/hosting of the docs site (CI publish workflow handled separately)
• Changes to the shared @nebari/starlight theme plugin itself
• Documenting the Helm chart, Tiltfile, or extensions/ internals in this pass