Add Pascal VOC dataset
_The documentation is not available anymore as the PR was closed or merged._ Some CI fails are unrelated to your PR and fixed on master, feel free to merge master into your branch :) Thanks @nateraw for the addition of this dataset.

I would suggest to transfer it to the Hugging Face Hub, under a "pascal" organization namespace: "pascal/voc".

What do you think? FYI I think this dataset is also available at (internal) https://huggingface.co/datasets/HuggingFaceM4/pascal_voc @lhoestq @albertvillanova what do you think best path forward is? No idea when I'll get to looking at this again, but would be nice to know plan so when I find time I can just get it done in one sitting.  My (not strong) opinion on this:
- as we are removing dataset scripts from GitHub, this dataset should be created directly on the Hub
- I proposed doing it under some kind of "official" org namespace, like pascal or pascal2; other suggestions are welcome
- the link given by @lhoestq might serve as inspiration for your implementation (I think yours misses data about action classification): their implementation comprises tasks: classification/detection, segmentation, action classification, person layout; it misses other tasks though

What do you think?
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.