[DOC]: `cuda::mr` needs docs

### 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

Merging `cuda::mr` in https://github.com/NVIDIA/libcudacxx/pull/309 was a major milestone towards providing a new, canonical C++ memory allocation API.

Sadly, no one can use it if they don't know about it. And no one will know about it if there aren't any docs :slightly_smiling_face:.

Relevant Links (internal)
- https://docs.google.com/presentation/d/1SI8KwgrGRIx2Tg9dz-RK5E63Ni6LwTU-o-7FEtXA2Xg/edit?usp=sharing
- https://docs.google.com/document/d/1ldc3REXESELjMYNfZc8H3vbPz84T-kDGOJ80KFi7Dy4/edit?usp=sharing

### 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_