feat: add Docker Sandbox deployment support

Add example and documentation for deploying sandbox-agent inside Docker
Sandbox microVMs for enhanced isolation on macOS/Windows.

- Add examples/docker-sandbox/ with TypeScript example
- Add docs/deploy/docker-sandbox.mdx with setup guide using custom templates
- Update docs navigation to include Docker Sandbox

examples/docker-sandbox/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "@sandbox-agent/example-docker-sandbox",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "start": "tsx src/docker-sandbox.ts",
+    "cleanup": "tsx src/cleanup.ts",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {},
+  "devDependencies": {
+    "@types/node": "latest",
+    "tsx": "latest",
+    "typescript": "latest",
+    "vitest": "^3.0.0"
+  }
+}