[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 docs are picking up and rendering the docstring for _iterators.IteratorBase which is an internal class. Instead, we should document e.g., CountingIterator, TransformIterator. etc., from iterators.py and ensure their docstrings are rendered.

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

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/