Add CCAgT dataset
_The documentation is not available anymore as the PR was closed or merged._ Hi! Excellent job @johnnv1! There were typos/missing words in the card, so I took the liberty to rewrite some parts to make them easier to understand. Let me know if you are ok with the changes. Also, feel free to add some info under the `Who are the annotators?` section.

Additionally, I fixed the issue with streaming and renamed the `digits` feature to `objects`.

@lhoestq Are you ok with skipping the dummy data test here as it's tricky to generate it due to the splits separation logic? I think I can also add instance segmentation: by exposing the segment of each instance, so it will be similar with object detection:

- `instances`: a dictionary containing bounding boxes, segments, and labels of the cell objects 
  - `bbox`: a list of bounding boxes
  - `segment`: a list of segments in format of `[polygon]`, where each polygon is `[x0, y0, ..., xn, yn]`
  - `label`: a list of integers representing the category

Do you think it would be ok? Don't you think it makes sense to keep the same category IDs for all approaches? 

Now we have:
 - nucleus category ID equals 0 for object detection and instance segmentation
 - background category ID equals 0 (on the masks) for semantic segmentation I find it weird to have a dummy label in object detection just to align the mapping with semantic segmentation. Instead, let's explain in the card (under Data Fields -> annotation) what the pixel values mean (background + object detection labels) Ok, I can do that in the next few days. I will create a `lapix` organization, and I will add this dataset and also #4565 So, I think we can close this PR? I have already moved these files there.

The link of CCAgT dataset is: https://huggingface.co/datasets/lapix/CCAgT

🤗  Woohoo awesome !

closing this PR :)
As described in #4075 

I could not generate the dummy data. Also, on the data repository isn't provided the split IDs, but I copy the functions that provide the correct data split. In summary, to have a better distribution, the data in this dataset should be separated based on the amount of NORs in each image.