Docs for creating an audio dataset
_The documentation is not available anymore as the PR was closed or merged._ Awesome thanks ! I think we can also encourage TAR archives as for image dataset scripts (feel free to copy paste some parts from there lol) Thanks for all the great feedback @polinaeterna and @lhoestq! 🥰

I added all the other feedback, and I'll look into the `librivox-indonesia` script now! If you don't mind, I'm taking over this PR since we'll do a release pretty soon @lhoestq no, I do :D  haha sorry ^^
This PR is a first draft of how to create audio datasets (`AudioFolder` and loading script). Feel free to let me know if there are any specificities I'm missing for this. 🙂