docs: update documentation for Halo 2.1 (#162)

为 Halo 2.1 更新文档。

see https://github.com/halo-dev/halo/releases/tag/v2.1.0

/kind documentation

```release-note
None
```

versioned_docs/version-2.1/developer-guide/theme/template-variables/post.md

@@ -0,0 +1,65 @@
+---
+title: 文章
+description: post.html - /archives/:slug
+---
+
+import CategoryVo from "../vo/CategoryVo.md"
+import TagVo from "../vo/TagVo.md"
+import ContentVo from "../vo/ContentVo.md"
+import Contributor from "../vo/Contributor.md"
+import PostVo from "../vo/PostVo.md"
+
+## 路由信息
+
+- 模板路径：`/templates/post.html`
+- 访问路径：`/archives/:slug`
+
+## 变量
+
+### post
+
+#### 变量类型
+
+[#PostVo](#postvo)
+
+#### 示例
+
+```html title="/templates/post.html"
+<article>
+  <h1 th:text="${post.spec.title}"></h1>
+  <div th:utext="${post.content.content}"> </div>
+</article>
+```
+
+### _templateId
+
+#### 变量值
+
+`post`
+
+## 类型定义
+
+### CategoryVo
+
+<CategoryVo />
+
+### TagVo
+
+<TagVo />
+
+### Contributor
+
+<Contributor />
+
+### ContentVo
+
+<ContentVo />
+
+### PostVo
+
+<PostVo />
+
+- [#CategoryVo](#categoryvo)
+- [#TagVo](#tagvo)
+- [#Contributor](#contributor)
+- [#ContentVo](#contentvo)