docs(llms): add QuickSilver Pro provider page

[raullenchai]

Summary

Adds a QuickSilver Pro provider page under aider/website/docs/llms/, following the same template as openrouter.md / deepseek.md. QuickSilver Pro is an OpenAI-compatible gateway for 12 frontier open-source models (DeepSeek V4 Flash/Pro, V3, R1, Qwen 3.5/3.6, Kimi K2.6, Gemini 2.5/3 family), priced ~20% below OpenRouter on the shared model set.

Setup that works today:

export OPENAI_API_BASE=https://api.quicksilverpro.io/v1
export OPENAI_API_KEY=<key>

aider --model openai/deepseek-v4-pro

Reasons it might be useful in the docs:

• DeepSeek V4 Pro is a strong aider pairing for multi-file refactors — competitive with Sonnet on SWE-bench at a fraction of the cost.
• Pricing runs ~20% below OpenRouter on the shared model set, so users coming over from OR get a margin bump without changing their workflow.
• Standard usage.cost is returned on every response, so aider's session-cost display works without configuration.

Disclosure

I'm the maintainer of QuickSilver Pro. Happy to drop this PR or rework the framing if a vendor-add isn't the right shape for the docs — alternatively, we'd be glad to host a tutorial under our own domain and you can link it however suits you.

Setup reference: https://quicksilverpro.io/docs/integrations/