[DOC]: cuda.parallel: Update documentation to include iterators and how to use them

[shwina]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this bug and that I agree to the Code of Conduct

Is this for new documentation, or an update to existing docs?

New

Describe the incorrect/future/missing documentation

Currently, the documentation shows how to call reduce_into with an array as input. We should additionally include examples of using iterators as inputs.

https://nvidia.github.io/cccl/cuda_parallel/

It would be good to include at least a few other examples:

• Reducing a CountingIterator
• Reducing a TransformIterator applied to a device array, to compute e.g., sum-of-squares
• Reducing a TransformIterator applied to another iterator (e.g., CountingIterator to compute e.g., 1**2 + 2**2 + 3**2 + ...).

If this is a correction, please provide a link to the incorrect documentation. If this is a new documentation request, please link to where you have looked.

https://nvidia.github.io/cccl/cuda_parallel/