Need to write reference documentation for xpert's API

[CRY0100]

Need to write reference documentation for xpert's API，This API documentation is intended to be provided for the button under the Agent section of our XPERT platform.

"This issue is a prerequisite for [xpert-ai/xpert#320]. Please notify that issue once it is resolved."

[code-xpertai]

自动生成的 PR 内容

由于沙箱环境无法直接推送到 GitHub，以下是完整的变更内容，可以手动创建 PR：

PR 标题

docs: add API Reference tab with OpenAPI configuration

PR 描述

## Summary

This PR adds API Reference documentation to the Xpert AI docs site, resolving issue #1.

### Changes

1. **Added API Reference navigation tabs**
   - English version: "API Reference" tab under AI product
   - Chinese version: "API 参考" tab under AI product

2. **Created API Reference overview pages**
   - `en/ai/api-reference/index.mdx` - English API documentation overview
   - `zh-Hans/ai/api-reference/index.mdx` - Chinese API documentation overview

3. **Configured OpenAPI specification**
   - Added `"api": { "openapi": "api/openapi.ai.json" }` to `docs.json`
   - This links to the existing OpenAPI specification file at `api/openapi.ai.json`

### API Documentation Coverage

The OpenAPI specification (`api/openapi.ai.json`) includes the following API categories:
- AI v1 API (Knowledge Base, File Operations, ChatKit Sessions)
- AI Contexts API
- AI Knowledges API  
- AI Assistants API
- AI Threads API (with streaming support)
- AI Conversations API (with feedback system)
- AI Store API

### Testing

- ✅ JSON validation passed for `docs.json`
- ✅ Navigation structure follows existing patterns
- ✅ API Reference tab appears as the first tab in AI product

Closes #1

变更文件

• docs.json - 添加 API 配置和导航
• en/ai/api-reference/index.mdx - 新建英文概述页面
• zh-Hans/ai/api-reference/index.mdx - 新建中文概述页面

分支信息

• 分支名: feature/add-api-reference
• 提交: 7cd8eaa docs: add API Reference tab with OpenAPI configuration