The guide Futures and Promises should be part of the Scala Book

[julienrf]

We would like the Scala Book to be a complete documentation of the language.

The guide Futures and Promises should be merged into the Scala Book. I think it should be added as new pages within the chapter Concurrency.

[LydiaSkuse]

there's a lot of duplicated information across the two pages and the Concurrency pages links to the Futures and Promises page a number of times. will probably need some careful content editing when moving and the links updating too.