[Documentation] Add a reference page / cookbook of common concurrency operations

[Daenyth]

It should cover when you'd use and how to use:

• parEvalMap/Unordered
• parJoin
• concurrently
• merge*

There's the page on Queue/Topic/etc but there's not a great single page on the operations directly on Stream. The Guide section on concurrency is very short and doesn't cover some of the most common operations (parEvalMap especially)