[docs] Per-layer config

[stevhliu]

Some updates and edits to improve the docs:

• update from "Heterogeneous model configurations" to "Per-layer configurations" so its less abstract, shorter and clearer
• lead with the problem and when to use per_layer_config so the motivation arrives faster for why to reach for this power feature

@eladsegal, if you could also take a look that'd be awesome 😄

[github-actions]

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[HuggingFaceDocBuilderDev]

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