docs: add documentation for author template variables (#176)

添加主题端作者归档页面的文档。后端改动可查阅：https://github.com/halo-dev/docs/issues/144

/kind documentation

Fixes #143 

```release-note
None
```

sidebars.js

@@ -128,6 +128,7 @@ module.exports = {
                 "developer-guide/theme/template-variables/tag",
                 "developer-guide/theme/template-variables/categories",
                 "developer-guide/theme/template-variables/category",
+                "developer-guide/theme/template-variables/author",
               ],
             },
             {