Document how to create a dataset loading script for audio/vision
Currently, in our docs for Audio/Vision/Text, we explain how to:
- Load data
- Process data

However we only explain how to *Create a dataset loading script* for text data.

I think it would be useful that we add the same for Audio/Vision as these have some specificities different from Text.

See, for example:
- #4697
  - and comment there: https://github.com/huggingface/datasets/issues/4697#issuecomment-1191502492

CC: @stevhliu 
