[FEA]: Update thread group concept in libcu++ docs

### Is this a duplicate?

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

### Area

Thrust

### Is your feature request related to a problem? Please describe.

This concept is dated https://nvidia.github.io/cccl/libcudacxx/extended_api/thread_groups.html

### Describe the solution you'd like

We should update the concept in libcu++ docs to use new thread group design

### Describe alternatives you've considered

_No response_

### Additional context

_No response_