Enrich documentation for `{iter_index}` variable in `workflow iterate`

[gavin-k-lee]

The documentation section for workflow iterate should include details of what form of script and command-line argument structure Renku expects to work with. The example given:

renku workflow iterate --map parameter-1=[10,20,30] --map output=output_{iter_index}.txt my-run

does not adequately describe the requirements on my-run. In particular, that

• output files need to be passed as command-line arguments to whatever script my-run ran
• my-run cannot hard-code output scripts.

An actual example here would be greatly appreciated.