Doc updates

[rahul003]

• Hook: 'main interface to use training'
• Glossary missing - "Glossary is in api.md"
• Some links broken
• Rule : A condition that will trigger an exception and terminate the training job early. Doesn't have to do that
• Hook from SageMaker?
• Api.md has SageMaker Zero-Code-Change vs. Python API, which is not on the page
• Built in rules table is broken
• Homepage: If a rule is triggered, it will raise a CloudWatch event and stop the training job, -> Incorrect
• Distributed training
• A section on keras (non tf.keras)
• Beef up PT and MXNet pages to be as good as TF
• Complete XGBoost docs (Edward)
• Link to analysis docs in main README