Improve image classification example
# What does this PR do?

This PR improves the image classification example script, by:
- replacing the `nateraw/imagefolder` hack with [`ImageFolder`](https://huggingface.co/docs/datasets/v2.0.0/en/image_process#imagefolder)
- improving the README, explaining how to easily train on custom data.
_The documentation is not available anymore as the PR was closed or merged._