Add a Markdown documentation site under docs/web using Astro content collections and Tailwind typography. It includes getting started, configuration, authentication, REST API, and Go SDK pages, with separate v0.2.0 and v0.3.0 snapshots and v0.3.0 as the default.
The layout has a compact sidebar with subsection highlighting, a native version selector, and automatic dark mode. Root redirects retain Go import metadata. Node.js 24 is included in the Nix development shell.
Validation: astro check passes without diagnostics; the static build generates 17 pages; docs checks pass for routes, links, version switching, snapshots, and Go import metadata.