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/docker-compose.yml

@@ -0,0 +1,35 @@
+services:
+  docs:
+    build:
+      context: .
+      target: ${APP_ENV:-dev}
+    restart: unless-stopped
+    environment:
+      - PUBLIC_DOMAIN=${PUBLIC_DOMAIN:-mcserial.l.zmesh.systems}
+      - VITE_HMR_HOST=${PUBLIC_DOMAIN:-mcserial.l.zmesh.systems}
+      - ASTRO_TELEMETRY_DISABLED=1
+    volumes:
+      # Dev mode: mount source for hot reload
+      - ./src:/app/src:ro
+      - ./public:/app/public:ro
+      - ./astro.config.mjs:/app/astro.config.mjs:ro
+    networks:
+      - caddy
+    labels:
+      # Caddy reverse proxy
+      caddy: ${PUBLIC_DOMAIN:-mcserial.l.zmesh.systems}
+      caddy.reverse_proxy: "{{upstreams 4321}}"
+
+      # WebSocket/HMR support for dev mode
+      caddy.reverse_proxy.flush_interval: "-1"
+      caddy.reverse_proxy.transport: "http"
+      caddy.reverse_proxy.transport.read_timeout: "0"
+      caddy.reverse_proxy.transport.write_timeout: "0"
+      caddy.reverse_proxy.transport.keepalive: "5m"
+      caddy.reverse_proxy.transport.keepalive_idle_conns: "10"
+      caddy.reverse_proxy.stream_timeout: "24h"
+      caddy.reverse_proxy.stream_close_delay: "5s"
+
+networks:
+  caddy:
+    external: true