add cudf to toctree
Follow up to https://github.com/huggingface/dataset-viewer/pull/2941

I realized the docs were not building in the CI: https://github.com/huggingface/dataset-viewer/actions/runs/9648374360/job/26609396615

Apologies for not checking this in my prior PR.

![Screenshot 2024-06-24 at 12 21 45 PM](https://github.com/huggingface/dataset-viewer/assets/17162724/9d7278b6-d00f-4795-93dc-259a08fb856b)

add cudf to toctree: Follow up to https://github.com/huggingface/dataset-viewer/pull/2941

I realized the docs were not building in the CI: https://github.com/huggingface/dataset-viewer/actions/runs/9648374360/job/26609396615

Apologies for not checking this in my prior PR.

![Screenshot 2024-06-24 at 12 21 45 PM](https://github.com/huggingface/dataset-viewer/assets/17162724/9d7278b6-d00f-4795-93dc-259a08fb856b)
