Add demo to docs website

[nvlang]

Add a fully functional demo to sveltex.dev.

Questions to answer:

• Could the Svelte REPL help with this?
• Can this be done within a Vue component (as would be needed to have it run inside VitePress)?
• Can something like this even be done within VitePress? Or should a separate website be deployed for the demo, hosted on a subdomain like demo.sveltex.dev or on a URL like sveltex.dev/demo?
• How can it be ensured that the demo is completely sandboxed, i.e., that it won't compromise the security of sveltex.dev even if malicious actors make use of it?

Aside from being a nice way for potential users to explore SvelTeX, a demo would also have the benefit of providing a simple way to share bug reproductions in issue reports.