[Docx] add language (en/cn) switch links (#4470)

* add install docs

* 修改文档

* 修改文档

docs/usage/code_overview.md

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+[简体中文](../zh/usage/code_overview.md)
+
 # Code Overview
 
 Below is an overview of the FastDeploy code structure and functionality organized by directory.

docs/usage/environment_variables.md

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+[简体中文](../zh/usage/environment_variables.md)
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 # FastDeploy Environment Variables
 
 FastDeploy's environment variables are defined in `fastdeploy/envs.py` at the root of the repository. Below is the documentation:

docs/usage/fastdeploy_unit_test_guide.md

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+[简体中文](../zh/usage/fastdeploy_unit_test_guide.md)
+
 # FastDeploy Unit Test Specification
 1. Test Naming Conventions
    - Test files must start with test_.

docs/usage/kunlunxin_xpu_deployment.md

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+[简体中文](../zh/usage/kunlunxin_xpu_deployment.md)
+
 ## Supported Models
 |Model Name|Context Length|Quantization|XPUs Required|Deployment Commands|Minimum Version Required|
 |-|-|-|-|-|-|

docs/usage/log.md

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+[简体中文](../zh/usage/log.md)
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 # Log Description
 
 FastDeploy generates the following log files during deployment. Below is an explanation of each log's purpose.