Audio/vision task guides
This PR adds fine-tuning guides for image classification, audio classification, and ASR since the APIs for these tasks are stable. Some general notes:

- The audio guides don't have TensorFlow sections yet.
- For image classification, there is a difference between the PyTorch and TensorFlow preprocessing methods. The TensorFlow example doesn't really add any transforms because I couldn't figure out how to integrate the Keras preprocessing/augmentation layers. ☹️ If we want to show some more substantial transforms, let me know and I can share my notebook or create an issue.
- Would be cool to add a widget to each guide so users can see the end result and what they can create!
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15808). All of your documentation changes will be reflected on that endpoint.