docs: document fused-kernel performance in Nunchaku Lite guide

[rootonchair]

What does this PR do?

Provide more information about the gain of fused kernel used in Nunchaku

Fixes # (issue)

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[stevhliu]

thanks!

[github-actions]

Hi @rootonchair, thanks for the PR! It does not appear to link an issue it fixes. If this PR addresses an existing issue, please add a closing keyword (e.g. Fixes #1234) to the PR description so the issue is linked. See the contribution guide for more details. If this PR intentionally does not fix a tracked issue, a maintainer can add the no-issue-needed label to silence this reminder.