Add docs for audio processing
Nice ! love it this way. I guess you can set this PR to "ready for review" ? I guess we can merge this one now :)
This PR adds documentation for the `Audio` feature. It describes:

- The difference between loading `path` and `audio`, as well as use-cases/best practices for each of them.
- Resampling audio files with `cast_column`, and then calling `ds[0]["audio"]` to automatically decode and resample to the desired sampling rate.
- Resampling with `map`.

Preview [here](https://52969-250213286-gh.circle-artifacts.com/0/docs/_build/html/audio_process.html), let me know if I'm missing anything!