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

[pahud]

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

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

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