docs(lambda-event-sources): Add documentation for Schema Registry integration with Kafka event sources

### Describe the issue

## Background

AWS Lambda recently added support for Schema Registry integration with Kafka event sources, allowing customers to use a schema registry to deserialize events from Kafka topics. This feature supports three Schema Registry types (Confluent Cloud, Confluent Self Managed, Glue Schema Registry) and three deserialization formats (Apache Avro, ProtoBuf, JSON).

### Issue

The AWS CDK implementation for this feature has been completed, but comprehensive documentation is needed so customers can discover and use the feature properly. Without proper documentation in the README files, customers will have difficulty understanding:

- How to set up Schema Registry integration
- Required IAM permissions for Glue Schema Registry
- Configuration options for different registry types
- Best practices for using Schema Registry with Lambda

### Proposed Solution

Add comprehensive documentation for Schema Registry integration to the following locations:

- The Lambda event sources `README.md`
- Usage examples for both ManagedKafkaEventSource and SelfManagedKafkaEventSource
- Clear notes about required fields and IAM permissions

### Value to Customers

This documentation will help customers:

1. Discover this new feature in CDK
2. Understand how to properly configure Schema Registry integration
3. Implement best practices for Lambda integration with Kafka and Schema Registry
4. Avoid common pitfalls and configuration errors

### Related PRs or Issues

- https://github.com/aws/aws-cdk/pull/34746

### Links

[aws-cdk-lib/aws-lambda-event-sources README](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda_event_sources-readme.html) - Main documentation for Lambda event sources

[aws-cdk-lib/aws-lambda API Reference](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_lambda-readme.html) - Documentation for the Lambda constructs, including the schema-registry related types

Review the Lambda event sources README and related PR #34746 first, then inspect the ManagedKafkaEventSource and SelfManagedKafkaEventSource usage examples and the referenced Lambda API documentation. Document Schema Registry setup, registry-specific configuration, required fields, Glue Schema Registry IAM permissions, and deserialization formats so customers can configure the integration without relying on undocumented details.

Written by the indexing model from the issue text.