[DOC]: Document feature macros

### 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

We are going to have feature macros, starting from #358. It would be nice to document them in a separate page in the docs.

This can help people use libcu++ in a forward/backward compatible way.

Discussion: https://github.com/NVIDIA/cccl/pull/358/files/4b9f0b8bb969646d6d877321e3d5de9da75bf833#r1318495726

### 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_