feat: add api-page doc

docs/en/guide/api-page.md

@@ -1 +1,22 @@
 # API Page
+
+The Island.js default theme has an built-in API page.
+
+Suppose your project has the following directory structure:
+
+```bash
+api
+├── config-basic.md
+├── config-extension.md
+└── index.md
+```
+
+Then you just need to add the following to `index.md` in the `api` directory:
+
+```md
+---
+pageType: api
+---
+```
+
+In this way, Island.js will automatically parse the document structure of other subpages in the directory for you, extract the title (h1, h2) information, and generate the API page.

docs/zh/guide/api-page.md

@@ -1 +1,22 @@
 # API 页面
+
+Island.js 默认主题内置了 API 页面。
+
+假如你的项目中有如下的目录结构:
+
+```bash
+api
+├── config-basic.md
+├── config-extension.md
+└── index.md
+```
+
+那么你只需要在 `api` 目录中的 `index.md` 中添加如下的内容即可:
+
+```md
+---
+pageType: api
+---
+```
+
+这样 Island.js 会自动为你解析该目录下其它子页面的文档结构，提取出其中的标题(h1、h2)信息，并生成 API 页面。