Doc: image_process small tip
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_The documentation is not available anymore as the PR was closed or merged._ This tip is unnecessary, i.e., Pillow will already be installed since the `Image` feature requires it for encoding and decoding. Thanks anyway.

cc @stevhliu I've noticed we are missing the installation section in the doc (`pip install datasets[vision]`). I can add it myself.
I've added a small tip in the `image_process` doc
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