[DOC]: Investigate docs grouping automation

### 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

As we transitioned to sphinx, we lost doxygen grouping automation (`@ingroup`). Now we have to manually maintain grouping pages in [thrust](https://github.com/NVIDIA/cccl/tree/main/docs/thrust/api_docs) and [cub](https://github.com/NVIDIA/cccl/blob/main/docs/cub/modules.rst). The disadvantage of this approach is that we have to remember to update docs as we modify sources.

We should investigate if some form or automated grouping is possible in our sphinx setup.

### 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_