Add docs link in sidebar, Dockerfile for VitePress, make Keycloak optional

- Add external "Docs" link in sidebar with configurable DOCS_URL
- Add Dockerfile.docs for VitePress static site (nginx)
- Add DOCS_URL build arg to Dockerfile.dashboard
- Run cargo fmt

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile.docs

@@ -0,0 +1,12 @@
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+COPY docs/package.json docs/package-lock.json ./
+RUN npm ci
+
+COPY docs/ .
+RUN npm run build
+
+FROM nginx:alpine
+COPY --from=builder /app/.vitepress/dist /usr/share/nginx/html
+EXPOSE 80