docs: add Cloudflare Sandbox SDK deployment guide

- Add docs/deploy/cloudflare.mdx with full deployment guide
- Add examples/cloudflare/ with working Worker code
- Update docs navigation to include Cloudflare option
- Update deploy index page with Cloudflare card

The example shows how to run sandbox-agent inside a Cloudflare Sandbox
with exposed ports for API access.

Co-authored-by: Shelley <shelley@exe.dev>

examples/cloudflare/wrangler.jsonc

@@ -0,0 +1,29 @@
+{
+  "$schema": "node_modules/wrangler/config-schema.json",
+  "name": "sandbox-agent-cloudflare",
+  "main": "src/cloudflare.ts",
+  "compatibility_date": "2025-01-01",
+  "compatibility_flags": ["nodejs_compat"],
+  "containers": [
+    {
+      "class_name": "Sandbox",
+      "image": "docker.io/cloudflare/sandbox:latest",
+      "instance_type": "lite",
+      "max_instances": 1
+    }
+  ],
+  "durable_objects": {
+    "bindings": [
+      {
+        "class_name": "Sandbox",
+        "name": "Sandbox"
+      }
+    ]
+  },
+  "migrations": [
+    {
+      "new_sqlite_classes": ["Sandbox"],
+      "tag": "v1"
+    }
+  ]
+}