Update training and fine-tuning docs (#1020)
* Update training and fine-tuning docs.
* Update examples README.
* Update README.
* Add Flax fine-tuning section.
* Accept suggestion
Co-authored-by: Anton Lozhkov <anton@huggingface.co>
* Accept suggestion
Co-authored-by: Anton Lozhkov <anton@huggingface.co>
Co-authored-by: Anton Lozhkov <anton@huggingface.co>
