Add Slack notification support for doc tests
# What does this PR do?

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This PR enables Slack reports for the daily doc tests.
_The documentation is not available anymore as the PR was closed or merged._<|||||>I will look this later today. Still learn this kind of things. If it is urgent, don't hesitate to go ahead though.<|||||>LGTM (without looking the notification script in detail).

Just 2 questions (due to my lack of expertise):

- why `working-directory: transformers` are removed (they appear in `self_scheduled.yml`)?
- I saw the following block is removed (it doesn't show up in `self_scheduled.yml` though)
```python
with:
      repository: 'huggingface/transformers'
      path: transformers
```
(just wondering why we don't need this)
<|||||>> 

Very good question! And I don't really know the answer :D I just noticed that it didn't work with 

```py
with:
      repository: 'huggingface/transformers'
      path: transformers
```

very well and I also think it's not necessary here so I removed it. Maybe @LysandreJik can give you a better answer.