Focus doc around preprocessing classes
This PR refocuses `preprocessing.mdx` around the preprocessor classes themselves instead of focusing on the different modalities. I think this puts more emphasis on what the library offers as opposed to focusing on something more generic like the different modalities. I know we're going to use `ImageProcessor` for image preprocessing soon, so I can update this again whenever that is ready.
_The documentation is not available anymore as the PR was closed or merged._