docs: update documentation for Halo 2.20 (#429)

为 [Halo 2.20](https://github.com/halo-dev/halo/releases/tag/v2.20.0) 更新文档。

/kind documentation

```release-note
None
```

docusaurus.config.js

@@ -28,7 +28,7 @@ const config = {
           routeBasePath: "/",
           showLastUpdateTime: true,
           showLastUpdateAuthor: true,
-          lastVersion: "2.19",
+          lastVersion: "2.20",
           versions: {
             current: {
               label: "2.20.0-SNAPSHOT",