docs(events): improved maintenance of information on supported targets (#30785)

### Reason for this change
Information about targets supported by `aws-events-targets` is also listed in `aws-events`, but this information is outdated and can be confusing to users.
Aggregating the list of supported targets in `aws-events-targets` will prevent contributors from forgetting to update it.

### Description of changes
Add a message to the README of `aws-events` encouraging people to look at `aws-events-targets` for information on targets.

### Description of how you validated changes



### Checklist
- [x] 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)

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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: sakurai-ryo

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

@@ -159,19 +159,7 @@ const ecsTaskTarget = new targets.EcsTask({ cluster, taskDefinition, role, platf
 The `aws-cdk-lib/aws-events-targets` module includes classes that implement the `IRuleTarget`
 interface for various AWS services.
 
-The following targets are supported:
-
-* `targets.CodeBuildProject`: Start an AWS CodeBuild build
-* `targets.CodePipeline`: Start an AWS CodePipeline pipeline execution
-* `targets.EcsTask`: Start a task on an Amazon ECS cluster
-* `targets.LambdaFunction`: Invoke an AWS Lambda function
-* `targets.SnsTopic`: Publish into an SNS topic
-* `targets.SqsQueue`: Send a message to an Amazon SQS Queue
-* `targets.SfnStateMachine`: Trigger an AWS Step Functions state machine
-* `targets.BatchJob`: Queue an AWS Batch Job
-* `targets.AwsApi`: Make an AWS API call
-* `targets.ApiGateway`: Invoke an AWS API Gateway
-* `targets.ApiDestination`: Make an call to an external destination
+See the README of the [`aws-cdk-lib/aws-events-targets`](https://github.com/aws/aws-cdk/tree/main/packages/aws-cdk-lib/aws-events-targets) module for more information on supported targets.
 
 ### Cross-account and cross-region targets