docs(langchain): create SiliconFlow provider entry

docs/docs/integrations/providers/siliconflow.mdx

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+# langchain-siliconflow
+
+This package contains the LangChain integration with SiliconFlow
+
+## Installation
+
+```bash
+pip install -U langchain-siliconflow
+```
+
+And you should configure credentials by setting the following environment variables:
+
+```bash
+export SILICONFLOW_API_KEY="your-api-key"
+```
+
+You can set the following environment variable to use the `.cn` endpoint:
+
+```bash
+export SILICONFLOW_BASE_URL="https://api.siliconflow.cn/v1"
+```
+
+## Chat Models
+
+`ChatSiliconFlow` class exposes chat models from SiliconFlow.
+
+```python
+from langchain_siliconflow import ChatSiliconFlow
+
+llm = ChatSiliconFlow()
+llm.invoke("Sing a ballad of LangChain.")
+```
+
+## Embeddings
+
+`SiliconFlowEmbeddings` class exposes embeddings from SiliconFlow.
+
+```python
+from langchain_siliconflow import SiliconFlowEmbeddings
+
+embeddings = SiliconFlowEmbeddings()
+embeddings.embed_query("What is the meaning of life?")
+```