Moving topic from old guide per CDK-753

I don't think we need to mention the stability of the FileSystem API. Our API uses it and we will update our internals for changes, insulating the user.

The relevant point is the second. The difference between using the HDFS API and Kite's API is that Kite is deliberately a high-level API, in terms of entities, views, and datasets. File management tasks are handled by Kite's internals so you don't have to. And, Kite has a single API that works with HDFS, HBase, and potentially others.