Adding object detection notebook to notebooks/examples

[MKhalusova]

Based on the object detection guide https://huggingface.co/docs/transformers/main/en/tasks/object_detection, this PR adds the same example notebook with PyTorch example of fine-tuning DETR for object detection. The text and description style have been modified to match the style of other example notebooks. The code is the same as in the doc.

[MKhalusova]

@NielsRogge Please let me know if more people need to be added as reviewers.