docs: add documentation for template engine global variables (#180)

添加 Halo 为模板引擎提供的全局变量的文档。

/kind documentation

Fixes #161 

```release-note
None
```

sidebars.js

@@ -151,6 +151,7 @@ module.exports = {
                 "developer-guide/theme/finder-apis/plugin",
               ],
             },
+            "developer-guide/theme/global-variables",
             "developer-guide/theme/code-snippets",
           ],
         },