Documentation: SageMaker distributed data parallel doc versioning

[TEChopra1000]

Description of changes:

• Adding a new folder to hold SageMaker's distributed data parallelism library's versioned API docs
• Modified the library's landing page to create uniformity with model parallelism docs

Testing done:
tox -e black-check,flake8,pylint,docstyle,sphinx,doc8 --parallel all

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[sagemaker-bot]

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• Commit ID: 6cfe54b
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[sagemaker-bot]

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AWS CodeBuild CI Report

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AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
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[ChaiBapchya]

Thanks!

[paul-cb]

Approved!