[DOC] - Update documentation engine to Astro

[andrewfulton9]

Preliminary Checks

• This issue is not a question, feature request, RFC, or anything other than a bug report. Please post those things in GitHub Discussions: https://github.com/nebari-dev/nebari/discussions

Summary

Add placeholder docs and framework for building and deploying:

• Use Astro as static site generator
• npm for dependency management
• utilizes nebari design components
• content is deployed to Cloudflare Pages via GitHub action
• includes instructions in README for updating docs
• update .gitignore

See nebari-dev/llm-serving-pack#120 as an example