[DOC]: Create a standalone page for Fancy iterators in CUB

### Is this a duplicate?

- [x] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

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

New

### Describe the incorrect/future/missing documentation

https://nvidia.github.io/cccl/cub/ has this near the top:

**> Utilities**
    **> Fancy iterators**

But nothing to click on, and neither "fancy" nor "iterator" appear anywhere else in that page.

### 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.

_No response_