[docs] optimization cleanup

[stevhliu]

cleans up the Optimization section a bit to remove redundant content

• removes the hardware specific sections (GPU/CPU) since many of the same optimizations in these docs already exist
• give Optimum it's own section in the docs for more visibility and combines GPU/CPU usage for it
• give assisted decoding it's own section in the docs for more visibility as well and it's also not really a decoding strategy like the other search/sampling methods are
• also removes the Optimizing inference doc since it also has redundant content in it and will be replaced by the newer overview in [docs] optimizations quickstart #42538

[HuggingFaceDocBuilderDev]

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[Cyrilvallez]

LGTM!