docs: update documentation for Halo 2.9 (#246)

为 Halo 2.9.0 更新文档。

/kind documentation

```release-note
None 
```

versioned_docs/version-2.9/developer-guide/theme/template-variables/page.md

@@ -0,0 +1,53 @@
+---
+title: 独立页面
+description: page.html - /:slug
+---
+
+import SinglePageVo from "../vo/SinglePageVo.md"
+import ContributorVo from "../vo/ContributorVo.md"
+import ContentVo from "../vo/ContentVo.md"
+
+## 路由信息
+
+- 模板路径：`/templates/page.html`
+- 访问路径：`/:slug`
+
+## 变量
+
+### singlePage
+
+#### 变量类型
+
+[#SinglePageVo](#singlepagevo)
+
+#### 示例
+
+```html title="/templates/page.html"
+<article>
+  <h1 th:text="${singlePage.spec.title}"></h1>
+  <div th:utext="${singlePage.content.content}"> </div>
+</article>
+```
+
+### _templateId
+
+#### 变量值
+
+`page`
+
+## 类型定义
+
+### SinglePageVo
+
+<SinglePageVo />
+
+- [#ContentVo](#contentvo)
+- [#ContributorVo](#contributorvo)
+
+### ContentVo
+
+<ContentVo />
+
+### ContributorVo
+
+<ContributorVo />