fix: correct NEXT_PUBLIC_DRAWIO_BASE_URL in offline deployment docs (#198)

- Fix wrong URL (http://drawio:8080 -> http://localhost:8080)
- Browser cannot resolve Docker internal hostnames
- Add docker-compose.yml example
- Simplify documentation

docker-compose.yml

@@ -0,0 +1,12 @@
+services:
+  drawio:
+    image: jgraph/drawio:latest
+    ports: ["8080:8080"]
+  next-ai-draw-io:
+    build:
+      context: .
+      args:
+        - NEXT_PUBLIC_DRAWIO_BASE_URL=http://localhost:8080
+    ports: ["3000:3000"]
+    env_file: .env
+    depends_on: [drawio]