Add tip on how to speed up loading with ImageFolder
_The documentation is not available anymore as the PR was closed or merged._ Thanks for adding that tip! 👍 

For the docs syntax, it might be better if we hide the package name/full path to the class or function and only show the name of it. I think it's easier for users to read the function name (eg,`cast_column`) instead of the full path which can be a bit lengthy for some functions like `datasets.IterableDataset.remove_columns` (and if we like this idea, we can align the rest of the docs on it).  > For the docs syntax, it might be better if we hide the package name/full path to the class or function and only show the name of it. I think it's easier for users to read the function name (eg,cast_column) instead of the full path which can be a bit lengthy for some functions like datasets.IterableDataset.remove_columns (and if we like this idea, we can align the rest of the docs on it).

That's also OK, as long as we are consistent.

@lhoestq @albertvillanova @polinaeterna Which one of these two styles do you prefer? Agree on hiding `datasets` name. Not sure about hiding class name as it's anyway not visible for users if they use `Dataset.cast_column` or  `IterableDataset.cast_column` when working with their datasets. But I agree that the most important thing is to be consistent :) Good points! :)

I think it'll be good to show the class name since some functions have different parameters. For example, if users click on `IterableDataset.map` and then `Dataset.map`, they'll see different parameters and have to figure out why (which isn't too difficult I guess lol). But showing the class name avoids any confusion upfront. 
This PR does two things:
* adds a tip on how to speed up loading of a large number of files with ImageFolder (motivated by [this issue](https://github.com/huggingface/datasets/issues/3960))
* replaces the current references to the `Dataset` methods in the Image Processing doc with their fully qualified counterparts (to align it with the Audio Processing doc)

cc @stevhliu 