[DOC]: Use doxygen tagfiles for better cross-project references

### 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?

Update

### Describe the incorrect/future/missing documentation

Each CCCL subproject has a separate Doxyfile, so the documentation parsers don't know about the other projects.

Doxygen offers "tagfiles" (https://stackoverflow.com/questions/15274074/what-exactly-is-in-a-doxygen-tagfile) that would allow our subprojects to be aware of each other.

See API-relevant issue here: https://github.com/NVIDIA/cccl/pull/8161#discussion_r2991199073

### 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_