Update training scripts docs
This PR updates the [Examples](https://huggingface.co/docs/transformers/examples) docs with examples for running a training script on distributed setups, with mixed precision, TPUs and Accelerate. It also adds examples for helpful options like using your own custom dataset, resume training from a checkpoint, and uploading to the Hub. 

The examples are centered around one task, but the user is expected to be able to generalize this guide to training scripts for other tasks as well. Feel free to let me know if I'm missing any major difference between the tasks that should be mentioned. I also created this guide as a new `mdx` file to be able to use the framework switcher, so we can remove `examples` from the toctree when we merge.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15931). All of your documentation changes will be reflected on that endpoint.