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/tokenizer_summary.mdx

@@ -104,6 +104,7 @@ seen before, by decomposing them into known subwords. For instance, the [`~trans
 
 ```py
 >>> from transformers import BertTokenizer
+
 >>> tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")
 >>> tokenizer.tokenize("I have a new GPU!")
 ["i", "have", "a", "new", "gp", "##u", "!"]
@@ -117,6 +118,7 @@ As another example, [`~transformers.XLNetTokenizer`] tokenizes our previously ex
 
 ```py
 >>> from transformers import XLNetTokenizer
+
 >>> tokenizer = XLNetTokenizer.from_pretrained("xlnet-base-cased")
 >>> tokenizer.tokenize("Don't you love 🤗 Transformers? We sure do.")
 ["▁Don", "'", "t", "▁you", "▁love", "▁", "🤗", "▁", "Transform", "ers", "?", "▁We", "▁sure", "▁do", "."]