ADD BORT
Hi,

this is a "clean" follow-up PR to the first attempt of adding Bort to Transformers (see #9112).

As Bort is based on the BERT architecture, there's no need to define own model classes, such as `BortModel`. This is done in the main Bort configuration via:

```json
"model_type": "bert"
```

Bort uses the same vocab as RoBERTa, so the tokenizer instance is also configured in the model configuration:

```json
"tokenizer_class": "RobertaTokenizer"
```

Basic integration tests and a (hopefully verbose) conversion script are also included in this PR.
> Thanks for adding this new model! When referencing other pages in the documentation, it's better to use `:doc:` instead of a hard link, as it will then work in all versions of the documentation (which don't have the same base url).

Sorry that was my bad! I copied it from DialoGPT -> Updated it there as well<|||||>Ah sorry @patrickvonplaten, the `model_doc/` should be removed are the pages are in the same folder. That should resolve the build doc error.<|||||>Great job @stefan-it 