Docs: add guide for customizing CDK stacks (e.g., adding DynamoDB with permissions)

## Description

There is no documentation to help users customize CDK stacks — for example, adding a custom DynamoDB table and granting the agent read-write permissions to it. Currently users have to figure this out on their own by:

1. Adding a stack in `agentcore/cdk/lib/dynamodb-stack.ts`
2. Referencing the stack in `agentcore/cdk/bin/cdk.ts`

This was not a straightforward process and should be documented.

## Steps to Reproduce

1. Try to add a custom AWS resource (e.g., DynamoDB table) to the CDK stack
2. Search the docs for guidance on customizing CDK stacks and wiring permissions

## Expected Behavior

Documentation should exist covering:
- How to add custom CDK stacks (e.g., DynamoDB, S3)
- How to reference new stacks in `cdk.ts`
- How to grant the agent IAM permissions to custom resources
- General guidance on interacting with and customizing the generated CDK code

## Actual Behavior

No documentation exists for CDK customization. Users must reverse-engineer the structure.

## CLI Version

_N/A_

## Operating System

macOS