Adding `audio-classification` example in the doc.
Author: Narsil
Pull request: yes

# What does this PR do?

Follow up of https://github.com/huggingface/transformers/pull/20226

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Fixes # (issue)


## 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/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20235). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20235). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20235). All of your documentation changes will be reflected on that endpoint.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20235). All of your documentation changes will be reflected on that endpoint.