SageMaker Studio QoL: prefixes, CLI flags, and docs

[oegedijk]

Summary

• Add SageMaker Studio auto-detection and default proxy prefixes in ExplainerDashboard.
• Add dash_kwargs passthrough and sagemaker handling in .run().
• Add CLI flags --sagemaker, --routes-pathname-prefix, --requests-pathname-prefix.
• Document SageMaker Studio workflow in README and note in release notes.

Notes

• Auto-detection uses /opt/ml/metadata/resource-metadata.json (plus env hints).
• CLI flags allow override if Studio proxy path differs.

Testing

• Not run (needs SageMaker Studio environment).