feat: 🎸 add an index
recommended by mongo atlas

<img width="1060" alt="Capture d’écran 2023-05-23 à 20 09 57" src="https://github.com/huggingface/datasets-server/assets/1676121/e6817023-40c0-443e-a590-90115b9eee6c">

feat: 🎸 add an index: recommended by mongo atlas

<img width="1060" alt="Capture d’écran 2023-05-23 à 20 09 57" src="https://github.com/huggingface/datasets-server/assets/1676121/e6817023-40c0-443e-a590-90115b9eee6c">
