Support spa route.

GitHub Pages  doesn't support `rewrites` like something in vercel
```json
{
  "rewrites": [
    {
      "source": "/(.*)",
      "destination": "/index.html"
    }
  ]
}
``` 

Maybe docup can support spa route?
It means for the following config:

```js
{
        text: "中文",
        link: "zh",
}
```

Docup will fetch `zh.md` directly instead of jump to `/zh`
