rename datasets-server to dataset-viewer
See https://github.com/huggingface/dataset-viewer/issues/2650

Tell me if it's OK, or if it's a breaking change that must be handled differently.

Also note that the docs page is still https://huggingface.co/docs/datasets-server/, so I didn't change it.

And the API URL is still https://datasets-server.huggingface.co/ (and [might always be](https://github.com/huggingface/dataset-viewer/issues/2666)), so I let it too.