Streams processing and YAML config

The documentation is incomplete as there are no clear examples on how to add streams processing to the YAML config. Only `streams_file` in `service` is documented where it also say that it can be added to `streams` section in the YAML config, but nowhere is it documented how that looks like.

Start with the documentation that describes `streams_file` under the `service` section and trace its reference to the YAML `streams` section. Add a clear configuration example showing how streams processing is declared there, and verify that the example matches the supported YAML structure.

Written by the indexing model from the issue text.