Doc styler examples (#14953)

* 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

docs/source/model_doc/longformer.mdx

@@ -75,8 +75,8 @@ For more information, please refer to the official [paper](https://arxiv.org/pdf
 trained and should be used as follows:
 
 ```python
-input_ids = tokenizer.encode('This is a sentence from [MASK] training data', return_tensors='pt')
-mlm_labels = tokenizer.encode('This is a sentence from the training data', return_tensors='pt')
+input_ids = tokenizer.encode("This is a sentence from [MASK] training data", return_tensors="pt")
+mlm_labels = tokenizer.encode("This is a sentence from the training data", return_tensors="pt")
 
 loss = model(input_ids, labels=input_ids, masked_lm_labels=mlm_labels)[0]
 ```