Add new model docs
# What does this PR do?

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This PR adds more information on how to add a model to Transformers docs.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

## UPDATE

The `model_doc/add_new_model.rst` is now finished for a first merge IMO. It would be amazing if @LysandreJik @sgugger you could review the file real quick again - I tried to add all of your suggestions. Also, I added a diagram showing the model design of Transformers - which was not reviewed yet. Note that I did not add a clear design for Tokenizers since it takes a lot of time to do so and I want to iteratively improve this step-by-step explanation. The first model, for which I'd like to mentor someone from the community would also be BigBird which does not need a new tokenizer.

In addition, I would be extremely grateful if @stas00 @abhishekkrthakur @patil-suraj @stefan-it @NielsRogge you have 10 minutes review the `model_doc/add_model.rst` file for possible improvements since you guys just recently added a new model. Your feedback would be especially useful since you might have a much more "unbiased" view what is difficult/easy when adding a model.