docs(cdk): updated README with toolkitBucket bootstrap configuration object (#24451)

The `--toolkit-bucket-name` and `--bootstrap-kms-key-id` are specified in `cdk.json` by passing a `toolkitBucket` [object](https://github.com/aws/aws-cdk/blob/3d7505b5a9c3daf5edfecdb4576b555bfe8d7553/packages/aws-cdk/lib/cli.ts#L503-L504).
The [documentation on the website](https://docs.aws.amazon.com/cdk/v2/guide/cli.html#cli-config) should also be updated.

Closes #24259 .

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: lpizzinidev

packages/aws-cdk/README.md

@@ -757,8 +757,11 @@ Some of the interesting keys that can be used in the JSON configuration files:
         "key": "value"
     },
     "toolkitStackName": "foo",        // Customize 'bootstrap' stack name  (--toolkit-stack-name=foo)
-    "toolkitBucketName": "fooBucket", // Customize 'bootstrap' bucket name (--toolkit-bucket-name=fooBucket)
-    "versionReporting": false,         // Opt-out of version reporting      (--no-version-reporting)
+    "toolkitBucket": {
+        "bucketName": "fooBucket",    // Customize 'bootstrap' bucket name (--toolkit-bucket-name=fooBucket)
+        "kmsKeyId": "fooKMSKey"       // Customize 'bootstrap' KMS key id  (--bootstrap-kms-key-id=fooKMSKey)
+    },
+    "versionReporting": false,        // Opt-out of version reporting      (--no-version-reporting)
 }
 ```