Add Pascal VOC dataset
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_4395). All of your documentation changes will be reflected on that endpoint.
This PR adds the Pascal VOC dataset in the same way TFDS has it added. I believe we can iterate on this dataset and in future versions include more data, such as segmentation masks, but for now I think it is a good idea to just add it the same way as TFDS to get a solid first version out there.