docs: update permalink in frontmatter (#25)

Author: Mister-Hope

docs/reference/frontmatter.md

@@ -109,14 +109,16 @@ layout: CustomLayout
 
 ## permalink
 
-- Type: `string`
+- Type: `string | null`
 
 - Details:
 
   Permalink for the page.
 
   This will override the default route path that determined by the file path of the page.
 
+  When it is set to `null`, the permalink of the page will be disabled.
+
 - Also see:
   - [Frontmatter > permalinkPattern](#permalinkpattern)
   - [Guide > Page > Routing](../guide/page.md#routing)

docs/zh/reference/frontmatter.md

@@ -109,14 +109,16 @@ layout: CustomLayout
 
 ## permalink
 
-- 类型： `string`
+- 类型： `string | null`
 
 - 详情：
 
   页面的永久链接。
 
   它将会覆盖根据文件路径来决定的默认路由路径。
 
+  当被设置为 `null` 时，将会禁用页面的永久链接。
+
 - 参考：
   - [Frontmatter > permalinkPattern](#permalinkpattern)
   - [指南 > 页面 > 路由](../guide/page.md#路由)