Update tutorial docs
This is a first draft of tutorial docs for the `pipeline`, `AutoClasses`, and how to preprocess data. Main updates include:

- How to use the `pipeline` for image and vision tasks.
- Highlight the `AutoFeatureExtractor` and `AutoProcessor` for image, vision and multimodal tasks.
- Add tutorials for how to preprocess audio, vision, and multimodal data. I'd also really appreciate it if @patrickvonplaten or @anton-l could take a look at the audio tutorial, and if @NielsRogge could take a look at the vision and multimodal tutorials (thank you so much for your notebooks! ❤️ ).

Once we have a concept guide section, I think we should move this [section](https://huggingface.co/docs/transformers/master/en/preprocessing#everything-you-always-wanted-to-know-about-padding-and-truncation) about padding/truncation there as well as the summary pages about the tasks and models.

The preprocess document is also quite long now, so please let me know if you'd like me to create a separate page for each modality. I think this would make it easier for users to read and find what they're looking for.
Thanks for the feedback y'all! I've also made a few additional changes:

- Rewrite multimodal tutorial with a different dataset (LJ Speech) because we can't use TIMIT.
- Add output image for preprocessed image so users can see what it looks like.

Let me know if there is anything else you'd like to see, otherwise I think we are ready to merge!