Add RedCaps dataset
Cool ! If you want you can include `dataset_infos.json` but only for the main configurations. That's what we do for example for translation datasets when there are too many configs @lhoestq I've added an example that uses `map` to download the images.
Add the RedCaps dataset. I'm not adding the generated `dataset_infos.json` file for now due to its size (11 MB). 

TODOs:
- [x] dummy data
- [x] dataset card

Close #3316 