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
https://docs.astro.build/en/guides/framework-components/

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

Show how to create fallback content to show while a client:only island is loading:

<A client:only="vue">
  <div slot="fallback">Loading</div>
</A>

Possibly a new H2 on the main UI frameworks page?

## Display loading content

Or something like that?

🖥️ Reproduction of code samples in StackBlitz

No response