[Docs] Link to dynamic lora from docs

[thomasjpfan]

Pull Request Description

Currently the link from VLLM_ALLOW_RUNTIME_LORA_UPDATING points to a non-existing page. With this PR, it now points to https://docs.vllm.ai/en/stable/features/lora.html#dynamically-serving-lora-adapters

Related Issues

None

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[Jeffwan]

@thomasjpfan thanks for the update. could you sign off the commit to pass DCO check?

[Jeffwan]

I will merge this one and please remember to sign off commits next time