docs: add CDK alternative implementation section to README (#125)

- Add Alternative Implementations section after Introduction
- Reference GenAI IDP Accelerator for AWS CDK repository
- Update Table of Contents with new section
- Provide clear guidance for CDK users

Co-authored-by: Rafał Pawłaszek <rpawlasz@amazon.com>

Author: rpawlaszek

README.md

@@ -8,6 +8,7 @@ SPDX-License-Identifier: MIT-0
 ## Table of Contents
 
 - [Introduction](#introduction)
+- [Alternative Implementations](#alternative-implementations)
 - [Key Features](#key-features)
 - [Architecture Overview](#architecture-overview)
 - [Quick Start](#quick-start)
@@ -29,6 +30,10 @@ https://github.com/user-attachments/assets/272b543b-e506-48ce-acc1-361422d22322
 
 White-glove customization, deployment, and integration support for production use cases is also available through [AWS Professional Services](https://aws.amazon.com/professional-services/).
 
+## Alternative Implementations
+
+**Prefer AWS CDK?** This solution is also available as [GenAI IDP Accelerator for AWS CDK](https://github.com/cdklabs/genai-idp), providing the same functional capabilities through AWS CDK constructs for customers who prefer Infrastructure-as-Code with CDK.
+
 ## Key Features
 
 - **Serverless Architecture**: Built entirely on AWS serverless technologies including Lambda, Step Functions, SQS, and DynamoDB