Add documentation for launching multiple versions of a task.

[cppwfs]

There is a section on how to deploy multiple versions of an app in a stream. While this is the same for task, we need a section in the task docs that discusses the same thing.

[cppwfs]

Updated docs and images for task registration in dataflow.spring.io to show version selection: spring-io/dataflow.spring.io#503

[cppwfs]

Resolved via spring-io/dataflow.spring.io@e54a3f5