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/main_classes/callback.mdx

@@ -79,12 +79,13 @@ class MyCallback(TrainerCallback):
     def on_train_begin(self, args, state, control, **kwargs):
         print("Starting training")
 
+
 trainer = Trainer(
     model,
     args,
     train_dataset=train_dataset,
     eval_dataset=eval_dataset,
-    callbacks=[MyCallback]  # We can either pass the callback class this way or an instance of it (MyCallback())
+    callbacks=[MyCallback],  # We can either pass the callback class this way or an instance of it (MyCallback())
 )
 ```