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/Makefile

@@ -0,0 +1,50 @@
+.PHONY: dev prod build logs stop clean restart shell local local-build
+
+.DEFAULT_GOAL := dev
+
+# Development mode with hot reload
+dev:
+	@echo "Starting mcserial docs in development mode..."
+	APP_ENV=dev docker compose up -d --build
+	@echo "Site available at https://$(shell grep PUBLIC_DOMAIN .env | cut -d= -f2)"
+	@sleep 2
+	docker compose logs -f
+
+# Production mode with static build
+prod:
+	@echo "Starting mcserial docs in production mode..."
+	APP_ENV=prod docker compose up -d --build
+	@echo "Site available at https://$(shell grep PUBLIC_DOMAIN .env | cut -d= -f2)"
+	@sleep 2
+	docker compose logs -f
+
+# Build without starting
+build:
+	docker compose build
+
+# View logs
+logs:
+	docker compose logs -f
+
+# Stop containers
+stop:
+	docker compose down
+
+# Clean up containers, images, volumes
+clean:
+	docker compose down -v --rmi local
+
+# Restart containers
+restart: stop dev
+
+# Shell into running container
+shell:
+	docker compose exec docs sh
+
+# Local development without Docker
+local:
+	npm run dev
+
+# Production build locally
+local-build:
+	npm run build && npm run preview