docs: add documentation for theme error template (#287)

添加主题错误模板的文档。

/kind documentation

Fixes https://github.com/halo-dev/docs/issues/144


```release-note
None
```

sidebars.js

@@ -179,6 +179,7 @@ module.exports = {
                 "developer-guide/theme/template-variables/categories",
                 "developer-guide/theme/template-variables/category",
                 "developer-guide/theme/template-variables/author",
+                "developer-guide/theme/template-variables/error",
               ],
             },
             {