[DOC]: Indicate header to include for all API reference pages

### 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

If I go to any API reference page, I would like to know which header files I need to include to get access to that API. This is often not available.

For example:

- https://nvidia.github.io/cccl/cub/api/structcub_1_1DeviceScan.html I can only see the relevant header by looking at code snippets

- https://nvidia.github.io/cccl/cub/api/namespacecub_1add0251c713859b8974806079e498d10a.html no header is mentioned

- https://nvidia.github.io/cccl/cub/api/structcub_1_1DeviceTransform.html no header is mentioned

These are all in cub, but the same applies in many thrust API reference pages too.

It would be great if the relevant header was prominent at the top (or near to the top) of the page.

### 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_