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
```

docusaurus.config.js

@@ -189,7 +189,7 @@ const config = {
       prism: {
         theme: darkCodeTheme,
         darkTheme: darkCodeTheme,
-        additionalLanguages: ["java","sql"],
+        additionalLanguages: ["java", "json", "sql"],
       },
       zoom: {
         selector: ".markdown :not(a) > img",