Update fine-tune docs
This is a first draft of the updated fine-tune tutorial. Main updates include:

- Preprocess a different dataset (users are probably very used to seeing `imdb` now) to include more diverse examples so users don't feel like they are doing the same thing over and over.
- Update the `Trainer` example to include evaluating accuracy before you fine-tune. It seems like a good practice to include evaluating metrics during training, so we should present it first. Let me know if this is not common or generally applicable, and we can revert it!
- Update Keras example to use `to_tf_dataset` to convert a dataset to a TensorFlow format (cc @Rocketknight1 please feel free to let me know if I'm missing anything!)
_The documentation is not available anymore as the PR was closed or merged._