Add how to use a fallback slot with client:only

[sarah11918]

📚 Subject area/topic

UI framework islands

📋 Suggested page

https://docs.astro.build/en/reference/directives-reference/#clientonly

📋 General description or bullet points (if proposing new content)

After the existing codeblock showing all the frameworks individually, show how to create fallback content to show while a client:only island is loading. I suggest adding something like:

#### Display loading content

For components that render only on the client, it is also possible to display fallback content while they are loading. Use `slot="fallback"` on any child element to create content that will be displayed only until your client component is available:

``` title="src/pages/index.astro" {2}
<ClientComponent client:only="vue">
  <div slot="fallback">Loading</div>
</ClientComponent>
```

Would accept a PR to the directives-reference page adding this!