Update PEFT doc

[cuichenx]

What does this PR do ?

Update PEFT doc

Collection: [Note which collection this PR will affect]

Changelog

• Mention support for all linear layers (Add LoRA support to all linear layers #7988)
• Update support matrix to include Gemma and Starcoder

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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Additional Information

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