Add: task guide for zero shot object detection
This PR adds a task guide for zero-shot object detection. Unlike other task guides, there is no fine-tuning and complex preprocessing of custom data. However, the task illustrates different ways of inferencing with OWL-ViT, such as using the pipeline, manual inference with text queries, manual inference for a batch of examples, and image-guided object detection. 
The task guide is based on [this notebook](https://github.com/huggingface/notebooks/blob/main/examples/zeroshot_object_detection_with_owlvit.ipynb) with some additions (e.g. pipeline example) and some modifications. 
_The documentation is not available anymore as the PR was closed or merged._