docs: add basic documentation for theme development (#102)

添加最基本的主题开发文档，仅编写从创建主题项目到最终预览的部分。

/kind documentation

```release-noe
None
```

sidebars.js

@@ -9,7 +9,7 @@
  Create as many sidebars as you want.
  */
 
- /** @type {import('@docusaurus/plugin-content-docs').SidebarsConfig} */
+/** @type {import('@docusaurus/plugin-content-docs').SidebarsConfig} */
 module.exports = {
   // By default, Docusaurus generates a sidebar from the docs folder structure
   // tutorialSidebar: [{type: 'autogenerated', dirName: '.'}],
@@ -69,6 +69,7 @@ module.exports = {
           label: "主题开发",
           items: [
             "developer-guide/theme/prepare",
+            "developer-guide/theme/structure",
             // "developer-guide/theme/config-files",
             // "developer-guide/theme/global-variable",
             // "developer-guide/theme/public-template-tag",