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/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "@sandbox-agent/example-cloudflare",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "wrangler dev",
+    "deploy": "wrangler deploy",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "@cloudflare/sandbox": "latest"
+  },
+  "devDependencies": {
+    "@cloudflare/workers-types": "latest",
+    "@types/node": "latest",
+    "typescript": "latest",
+    "vitest": "^3.0.0",
+    "wrangler": "latest"
+  }
+}