aws-cdk: Feature flags documentation is gone

### Describe the issue

The [CDK documentation](https://docs.aws.amazon.com/cdk/v2/guide/featureflags.html) refers to [FEATURE_FLAGS.md](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/cx-api/FEATURE_FLAGS.md) which has been deleted from the repo in [this commit](https://github.com/aws/aws-cdk/commit/5e95ba2b7a24b2598cf00890e1e7d569914f27c9).

Please restore the documentation or update the docs to point to the new location.

### Links

The [CDK documentation](https://docs.aws.amazon.com/cdk/v2/guide/featureflags.html) refers to [FEATURE_FLAGS.md](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/cx-api/FEATURE_FLAGS.md) which has been deleted from the repo in [this commit](https://github.com/aws/aws-cdk/commit/5e95ba2b7a24b2598cf00890e1e7d569914f27c9).

Start with the linked CDK feature flags documentation and the referenced packages/@aws-cdk/cx-api/FEATURE_FLAGS.md path, then inspect commit 5e95ba2b7a24b2598cf00890e1e7d569914f27c9 to identify the replacement location. Done means the documentation links to an existing feature flags reference or the missing documentation is restored.

Written by the indexing model from the issue text.