Astro docs migration

[andrewfulton9]

Reference Issues or PRs

Closes #22

What does this implement/fix?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Summary

Migrates the docs site from Docusaurus to Astro + Starlight, matching the pattern established in nebari-dev/nebi-pack#40 and nebari-dev/llm-serving-pack#120, per the checklist in #22.

• Astro + Starlight replace Docusaurus as the static site generator (npm still used for dependency management, unchanged).
• Adopts the nebari-design token/component system: Tailwind v4, a shadcn-style components.json registry, and shared Button/Spinner/RepoButton/SocialIcons components.
• Adds a remark-base-links plugin (with unit tests) so internal links work correctly both at / and under a deployed subpath.
• Migrates existing content (Introduction, Authentication Flow, NebariApp CRD Reference) into the new Astro content collection — prose is unchanged, only frontmatter was adapted to Starlight's schema.
• Rewrites docs/README.md with the new npm commands (dev/build/preview/test) and testing/link-checking instructions.
• Adds scripts/check-links.sh, an internal link checker run in CI and available locally.
• Replaces the GitHub Pages deploy workflow with a docs.yml workflow that builds, unit-tests, link-checks, and deploys to Cloudflare Pages (with PR preview URLs and a cleanup workflow for closed PRs).
• Updates the top-level .gitignore for Astro build artifacts (docs/dist/, docs/.astro/) in place of the old Docusaurus entries.

Out of scope (intentionally): this repo has no top-level Makefile (only dev/Makefile, for the unrelated Tilt-based operator dev loop), so no make docs-* targets were added and the top-level README.md was left untouched — doc-update instructions live entirely in docs/README.md.

Testing

• Did you test the pull request locally?
• Did you add new tests?

npm install, npm test (10/10 passing), npm run build, and bash scripts/check-links.sh all pass locally. CI runs the same checks and additionally deploys to Cloudflare Pages (requires the CLOUDFLARE_API_TOKEN/CLOUDFLARE_ACCOUNT_ID repo secrets to be configured).

Documentation

docs/README.md documents the new install/dev/build/preview/test/link-check workflow and CI.

Access-centered content checklist

Text styling

• The content is written with plain language (where relevant).
• If there are headers, they use the proper header tags (with only one level-one header: H1 or # in markdown).
• All links describe where they link to (for example, check the Nebari website).
• This content adheres to the Nebari style guides.

Non-text content

• All content is represented as text (for example, images need alt text, and videos need captions or descriptive transcripts).
• If there are emojis, there are not more than three in a row.
• Don't use flashing GIFs or videos.
• If the content were to be read as plain text, it still makes sense, and no information is missing.

Any other comments?

None of the doc page prose changed — only the site generator, styling, and CI/deploy mechanism.