Docs: Separate documentation for mbart
Currently all mbart documentation is stuffed into docs/
https://github.com/huggingface/transformers/blob/353b8f1e7a7361c0afd9e391381bc226b4a5ca8f/docs/source/model_doc/bart.rst#L42

mbart should have it's own `model_doc/mbart.rst` and entry in `pretrained_models.rst`.



Optionally you can also create a new `src/transformers/modeling_mbart.py` with roughly these contents:

```python
from .modeling_bart import BartForConditionalGeneration
from .configuration_bart import MbartConfig
class MBartForConditionalGeneration(BartForConditionalGeneration):
    config_class = MbartConfig
	# this model fully inherits its implementation from bart
```
@sshleifer  I think adding new class should make thing more clear, should I go ahead with it ? Will also need to modify tests a little bit I guess<|||||>yes, thanks!

key tests not to break
```bash
RUN_SLOW=1 pytest tests/test_modeling_mbart.py
```<|||||>You can also make `tokenization_mbart.py`