Fix docs on audio feature installation
_The documentation is not available anymore as the PR was closed or merged._
This PR:
- Removes the explicit installation of `librosa` (this is installed with `pip install datasets[audio]`
- Adds the warning for Linux users to install manually the non-Python package `libsndfile`
- Explains that the installation of `torchaudio` is only necessary to support loading audio datasets containing MP3 audio files

Related to #4000.