Add MANtIS dataset
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4695). All of your documentation changes will be reflected on that endpoint.
This PR adds MANtIS dataset.
Arxiv: [https://arxiv.org/abs/1912.04639](https://arxiv.org/abs/1912.04639)
Github: [https://github.com/Guzpenha/MANtIS](https://github.com/Guzpenha/MANtIS)

README and dataset tags are WIP.