Moving Kite Dataset API info from old guide per CDK-753.

Link to the old guide from which this came. This is just shards of information brought forward so they're not lost. This info should be woven into the samples before 1.0.

http://kitesdk.org/docs/current/kite-data/guide.html
I think this guide should start by introducing `Dataset<EntityType>` and then `Datasets` methods that interact with `Datasets`. We should assume that users are familiar with creating a dataset via CLI, so we should start with opening an existing dataset.