docs: documentation for Halo 2.0 (#92)

提供 Halo 2.0 的文档。

TODO list:

- [x] Halo 2.0 的配置文档
- [ ] ~~插件/主题开发的简要文档~~

```release-note
提供 Halo 2.0 的文档
```

docusaurus.config.js

@@ -34,6 +34,13 @@ const config = {
           showLastUpdateAuthor: true,
           remarkPlugins: [math, mermaid],
           rehypePlugins: [katex],
+          lastVersion: "1.5.4",
+          versions: {
+            current: {
+              label: "2.0.0-SNAPSHOT",
+              path: "2.0.0-SNAPSHOT",
+            },
+          },
         },
         blog: false,
         theme: {