Notebook examples grouping and update
This PR groups the notebook examples on [this page](https://huggingface.co/docs/transformers/main/en/notebooks) by modality for easier navigation. It also adds a few notebooks from the official repo that were not previously listed, e.g. fine-tuning models for image classification, semantic segmentation, video classification, image similarity, and time series. 


_The documentation is not available anymore as the PR was closed or merged._