Update index.mdx with missing import

Added the missing import `import * as cdk from 'aws-cdk-lib';` to the page.

src/pages/[platform]/ai/conversation/knowledge-base/index.mdx

@@ -115,6 +115,7 @@ import { defineBackend } from '@aws-amplify/backend';
 import { auth } from './auth/resource';
 import { data } from './data/resource';
 import { PolicyStatement } from 'aws-cdk-lib/aws-iam';
+import * as cdk from 'aws-cdk-lib';
 
 const backend = defineBackend({
   auth,