Relocates the entire Docusaurus site from the repo root into a
dedicated docs/ directory to keep docs tooling separate from the
Rust/frontend workspace. Updates CI workflows (build.yml, docs.yml),
justfile (adds `just docs` task), and GEMINI.md to reflect the new
paths.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>