[train][doc] Update Ray Train example titles

[justinvyu]

Why are these changes needed?

This makes the example titles consistent and removes unnecessary information (ex: "mnist classifier" / "fashion mnist classifier" --> "image classifier"). Also, re-iterate the main value of running these examples with Ray Train at the top of the page.

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    method in Tune, I've added it in doc/source/tune/api/ under the
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[woshiyyya]

Thanks for polishing the example titles.