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`

Start by tracing how the configured link `{ text: "中文", link: "zh" }` is handled and how document routes are resolved. Check the existing navigation and document-loading tests, if present. Done means the `zh` link fetches `zh.md` directly instead of navigating to `/zh`, without breaking existing routes.

Written by the indexing model from the issue text.