GitLab

* 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