Add Starlight documentation site with full content

Astro 5 + Starlight docs site with 17 pages covering all 30 mcserial
tools. Structured using Diataxis framework: tutorials, guides, reference,
and concepts. Includes Docker + caddy-docker-proxy deployment for
mcserial.l.zmesh.systems with HMR support behind reverse proxy.

Content pages:
- Landing page with feature overview and quick start
- Tutorials: getting started, loopback testing (loop://)
- Guides: RS-232 basics, RS-485/Modbus, file transfers, network ports
- Reference: common tools, RS-232 tools, RS-485 tools, file transfer
  tools, URL schemes, MCP resources, environment variables
- Concepts: RS-232 vs RS-485, flow control

docs/Dockerfile

@@ -0,0 +1,48 @@
+# syntax=docker/dockerfile:1
+
+# ============================================
+# Base stage — Node 22 slim
+# ============================================
+FROM node:22-slim AS base
+
+WORKDIR /app
+COPY package.json package-lock.json ./
+
+# ============================================
+# Development — hot reload
+# ============================================
+FROM base AS dev
+
+RUN npm ci
+COPY . .
+
+EXPOSE 4321
+CMD ["npm", "run", "dev", "--", "--host", "0.0.0.0"]
+
+# ============================================
+# Build — static site generation
+# ============================================
+FROM base AS build
+
+RUN npm ci
+COPY . .
+
+ENV ASTRO_TELEMETRY_DISABLED=1
+RUN npm run build
+
+# ============================================
+# Production — Caddy serves static files
+# ============================================
+FROM caddy:2-alpine AS prod
+
+COPY --from=build /app/dist /usr/share/caddy
+
+RUN echo ':80 { \
+    root * /usr/share/caddy \
+    encode gzip \
+    try_files {path} {path}/ /index.html \
+    file_server \
+}' > /etc/caddy/Caddyfile
+
+EXPOSE 80
+CMD ["caddy", "run", "--config", "/etc/caddy/Caddyfile"]