Showcase registry

Now we manually add showcases to a particular page, it would be nice to have one config for showcase which will be used to inject a link to it to every related page.

Something like this 👇

```ts
// showcase/react-create-query/docs.ts

export default {
name: "React + createQuery",
articles: [
"/api/factories/create-query/",
"/integrations/react/use-query/"
]
}
```