docs: reorganize documentation into i18n folder structure (#466)

* docs: reorganize docs into en/cn/ja folders

- Move documentation files into language-specific folders (en, cn, ja)
- Add Chinese and Japanese translations for all docs
- Extract Docker section from README to separate doc file
- Update README to link to new doc locations

* docs: fix links to new docs folder structure

* docs: update README and provider docs

* docs: fix broken import statements in cloudflare deploy guides

* docs: sync CN/JA READMEs with EN structure and fix all paths

docs/en/offline-deployment.md

@@ -0,0 +1,39 @@
+# Offline Deployment
+
+Deploy Next AI Draw.io offline by self-hosting draw.io to replace `embed.diagrams.net`.
+
+**Note:** `NEXT_PUBLIC_DRAWIO_BASE_URL` is a **build-time** variable. Changing it requires rebuilding the Docker image.
+
+## Docker Compose Setup
+
+1. Clone the repository and define API keys in `.env`.
+2. Create `docker-compose.yml`:
+
+```yaml
+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]
+```
+
+3. Run `docker compose up -d` and open `http://localhost:3000`.
+
+## Configuration & Critical Warning
+
+**The `NEXT_PUBLIC_DRAWIO_BASE_URL` must be accessible from the user's browser.**
+
+| Scenario | URL Value |
+|----------|-----------|
+| Localhost | `http://localhost:8080` |
+| Remote/Server | `http://YOUR_SERVER_IP:8080` |
+
+**Do NOT use** internal Docker aliases like `http://drawio:8080`; the browser cannot resolve them.
+