docs(langchain): create SiliconFlow provider entry

[jackalcooper]

SiliconFlow's provider integration will be maintained at https://github.com/siliconflow/langchain-siliconflow
This PR introduce the basic instruction to make use of the pip package

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
langchain	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 31, 2025 8:13am

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #32342 will degrade performances by 10.07%

_{Comparing siliconflow:add-siliconflow-md (4d8a38e) with master (32e5040)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

❌ 1 regressions
✅ 12 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_import_time[ChatPromptTemplate]	409.2 ms	455 ms	-10.07%

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32342 will not alter performance

_{Comparing siliconflow:add-siliconflow-md (4d8a38e) with master (32e5040)}

Summary

✅ 14 untouched benchmarks