docs(apigatewayv2-integrations): add StepFunctions Integration to table of contents (#30387)

### Issue # (if applicable)

None

### Reason for this change

The index of `Stepfunctions Integration` is missed.

### Description of changes

```
- [HTTP APIs](#http-apis)
  - [Lambda Integration](#lambda)
  - [HTTP Proxy Integration](#http-proxy)
  - [StepFunctions Integration](#stepfunctions-integration) // added
```

### Description of how you validated changes

None

### 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)

----

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

Author: badmintoncryer

packages/aws-cdk-lib/aws-apigatewayv2-integrations/README.md

@@ -5,6 +5,7 @@
 - [HTTP APIs](#http-apis)
   - [Lambda Integration](#lambda)
   - [HTTP Proxy Integration](#http-proxy)
+  - [StepFunctions Integration](#stepfunctions-integration)
   - [Private Integration](#private-integration)
   - [Request Parameters](#request-parameters)
 - [WebSocket APIs](#websocket-apis)