docs(installation): update model_config examples and clarify vLLM backend setup

[samzong]

FILL IN THE PR DESCRIPTION HERE

FIX #675 (link existing issues this PR will resolve)

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 website

Owners: @Xunzhuo, @rootfs, @yuluo-yx
Files changed:

• website/docs/installation/configuration.md
• website/docs/installation/installation.md

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🎉 Thanks for your contributions!

This comment was automatically generated based on the OWNER files in the repository.

[samzong]

@yuezhu1 @Xunzhuo PTAL.

[yuezhu1]

Mostly good to me. Just nitpicking

[yuezhu1]

LGTM