docs(appsync): add placeholder Query type for improved schema compatibility (#32652)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

My earlier PR (#32633) addressing the same issue was approved but encountered sync problems, leading me to close it and open this fresh PR. I’ve ensured this branch is properly synced, and I hope the issue is resolved.

For clarity, the rationale behind this change is as follows:

While the current schema is technically valid with a single root Mutation type, most GraphQL clients and tools expect a default Query operation for introspection and standard queries. This PR introduces a minimal placeholder Query type to enhance compatibility and align with widely accepted GraphQL conventions.



### Description of changes



### Describe any new or updated permissions being added

<!— What new or updated IAM permissions are needed to support the changes being introduced ? -->


### Description of how you validated changes



### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: azarboon

packages/aws-cdk-lib/aws-appsync/README.md

@@ -240,6 +240,10 @@ input DemoInput {
 type Mutation {
   callStepFunction(input: DemoInput!): job
 }
+
+type Query {
+  _placeholder: String
+}
 ```
 
 GraphQL request mapping template `request.vtl`: