docs: restructure theme development documentation (#450)

重构主题开发的部分文档，合并模板路由和模板变量章节。

/kind documentation

```release-note
None
```

sidebars.js

@@ -320,13 +320,12 @@ module.exports = {
         "developer-guide/theme/prepare",
         "developer-guide/theme/config",
         "developer-guide/theme/structure",
-        "developer-guide/theme/template-route-mapping",
         "developer-guide/theme/static-resources",
         "developer-guide/theme/settings",
         "developer-guide/theme/annotations",
         {
           type: "category",
-          label: "模板变量",
+          label: "模板编写",
           link: {
             type: "doc",
             id: "developer-guide/theme/template-variables",