chore(ecs): add documentation to clarify that ECS clusters and their capacity providers (#35218)

Document ECS cross-stack capacity provider limitation

### Issue # (if applicable)

Closes #35210.

### Reason for this change

Users encounter circular dependency errors when trying to add ECS capacity providers from different stacks. The AWS CDK framework automatically creates bidirectional dependencies in cross-stack scenarios:

1. **Forward Dependency**: `ClusterStack` → `CapacityStack` (cluster needs capacity provider reference)
2. **Reverse Dependency**: `CapacityStack` → `ClusterStack` (capacity provider instances need cluster configuration)

This creates a circular dependency that CloudFormation cannot resolve. The current documentation does not clearly communicate this limitation, leading to user confusion.

### Description of changes

Added clear documentation notes in the ECS README to communicate that cross-stack ECS capacity provider registration is not supported by the CDK framework:

- Added a note in the main Capacity Providers section referencing the limitation
- Added a detailed note in the Auto Scaling Group Capacity Providers section explaining the constraint
- Provides clear guidance to deploy clusters and capacity providers in the same stack

This is a documentation-only change that clarifies existing behavior rather than introducing new functionality or breaking changes.

### Describe any new or updated permissions being added

No new or updated IAM permissions are needed - this is a documentation-only change.

### Description of how you validated changes

- Verified the documentation renders correctly in markdown
- Confirmed no code changes were made that could affect functionality
- Existing tests continue to pass unchanged

### 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: pahud

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

@@ -1591,6 +1591,8 @@ it in the constructor. Then add the Capacity Provider to the cluster. Finally,
 you can refer to the Provider by its name in your service's or task's Capacity
 Provider strategy.
 
+> **Note**: Cross-stack capacity provider registration is not supported. The ECS cluster and its capacity providers must be created in the same stack to avoid circular dependency issues.
+
 By default, Auto Scaling Group Capacity Providers will manage the scale-in and
 scale-out behavior of the auto scaling group based on the load your tasks put on
 the cluster, this is called [Managed Scaling](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/asg-capacity-providers.html#asg-capacity-providers-managed-scaling). If you'd