Add Donut image processor
# What does this PR do?

Adds an image processor for Donut. 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._<|||||>I'm currently unable to create a PR myself but please note that `DonutImageProcessor.preprocess()` still calls `.pad()` instead of `.pad_image()`, generating a *lot* of logger noise...<|||||>@pasky Thanks for raising! This should now be resolved with the merging of #20904 