[docs] showcase example of SSR with VM module

[augustjk]

Thanks @KonnorRogers for suggesting!

We can also showcase a running example of SSR with VM module, a la https://stackblitz.com/edit/lit-ssr-vm-module?file=src%2Fserver.js

While making this, I noticed the example in https://lit.dev/docs/ssr/server-usage/#vm-module is different than the example in https://github.com/lit/lit/tree/main/packages/labs/ssr#rendering-in-a-separate-vm-context

They ultimately do the same thing but looks like the lit.dev example was changed in #1026 since renderModule actually creates a module loader with some DOM shim baked into the global which no longer became necessary. (This actually might be some cruft we should address in the SSR package).