docs(README): Add API documentation links across multiple languages

- Add API documentation section to `README.md` with link to Apifox
- Add API documentation section to `README_ja.md` with Japanese translation and link to Apifox
- Add API documentation section to `README_cn.md` with Chinese translation and link to Apifox

Author: okatu-loli

README.md

@@ -101,6 +101,10 @@ English | [中文](./README_cn.md) | [日本語](./README_ja.md) | [Contributing
 
 <https://alistgo.com/>
 
+## API Documentation (via Apifox):
+
+<https://alist-public.apifox.cn/>
+
 ## Demo
 
 <https://al.nn.ci>

README_cn.md

@@ -99,6 +99,10 @@
 
 <https://alistgo.com/zh/>
 
+## API 文档（通过 Apifox 提供）
+
+<https://alist-public.apifox.cn/>
+
 ## Demo
 
 <https://al.nn.ci>

README_ja.md

@@ -100,6 +100,10 @@
 
 <https://alistgo.com/>
 
+## APIドキュメント（Apifox 提供）
+
+<https://alist-public.apifox.cn/>
+
 ## デモ
 
 <https://al.nn.ci>