Replace README-heavy docs with a proper docs site (Astro Starlight)

[PAMulligan]

Summary

Stand up a dedicated documentation site with a getting-started flow, configuration reference, deployment guides per host, plugin/tool authoring tutorials, and migration notes between versions.

Motivation

The current README is dense and growing fast. A docs site with search, versioned content, and per-topic pages scales better and signals product maturity to enterprise evaluators.

Acceptance Criteria

• docs/ Astro Starlight (or Docusaurus) project deployed to Cloudflare Pages at docs.claudius.dev (or equivalent)
• Top-level sections: Getting Started, Configuration, Deployment, Plugins & Tools, RAG, Channels, API Reference, Migration Guides, FAQ
• Versioned docs: each major release archived
• Built-in search (Pagefind or Algolia DocSearch)
• Embed the live demo widget on the home page so visitors can interact immediately
• README slimmed to a quick-start + link to the docs site