Migrate MXNet documentation to sphinx project

[eslesar-aws]

The initial incremental move of docs from readme to the sphinx project.

• created a new rst file using_mxnet.rst where I copied the existing content of src/mxnet/readme.rst
• edited that content (roughly first half, including creating links to class and method documentation on first mention within a subtopic--open to feedback on this approach.
• minor edits to index.rst and sagemaker.mxnet.rst to incorporate the new content in sphinx

Merge Checklist

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• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[laurenyu]

maybe "Classes" instead?

[laurenyu]

please update the PR title/commit message to an imperative statement (reference: How to Write a Git Commit Message)