Broadcast data tutorial needs to explain how to download the blob

[nguyer]

We have a great tutorial that explains how to upload and broadcast a blob (https://hyperledger.github.io/firefly/gettingstarted/broadcast_data.html). Usually the very next question is "How do I, or how does another member of the network download that blob? This is not covered in the guide, but can easily be accomplished with the /namespaces/{ns}/data/{dataid}/blob endpoint (https://hyperledger.github.io/firefly/swagger/swagger.html#/default/getDataBlob).

We should enhance this guide to add more detail on how to download files.