doc: Streams API Doc Rewrite

The Streams API doc is now broken up into 3 sections:

1. API for Consumers
2. API for Implementors
3. Details and Extras

This addresses one of the biggest points of confusion for new users who start to consume streams, and get the impression that they have to do lots of extra work and implement classes and such, just to get some data out of a file.

The easiest way to review is probably to just read the new doc:

https://github.com/isaacs/node/blob/doc-stream-refactor/doc/api/stream.markdown