* Added missing code in exemplary notebook - custom datasets fine-tuning
Added missing code in tokenize_and_align_labels function in the exemplary notebook on custom datasets - token classification.
The missing code concerns adding labels for all but first token in a single word.
The added code was taken directly from huggingface official example - this [colab notebook](https://github.com/huggingface/notebooks/blob/master/transformers_doc/custom_datasets.ipynb).
* Changes requested in the review - keep the code as simple as possible
* Fix bad examples
* Add black formatting to style_doc
* Use first nonempty line
* Put it at the right place
* Don't add spaces to empty lines
* Better templates
* Deal with triple quotes in docstrings
* Result of style_doc
* Enable mdx treatment and fix code examples in MDXs
* Result of doc styler on doc source files
* Last fixes
* Break copy from
* Convert a few docs
* And another
* Last tutorials
* New syntax for colab links
* Convert a few docs
* And another
* Last tutorials
* New syntax for colab links
