[docs] peft

[stevhliu]

updates the peft docs:

• a more complete training section with a full code snippet, describe saving behavior, resuming from a checkpoint, and distributed training
• adds some undocumented API methods (delete_adapter, active_adapters)
• describe loading a bit more (from state dict and how from_pretrained auto-detects an adapter)
• update API reference page
• better section structure and flow

[HuggingFaceDocBuilderDev]

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