doc: Java configuration block in "Extending Akka"

[leviramsey]

Kind of weird that this renders as

ruby akka.actor.typed { extensions = ["jdocs.akka.extensions.ExtensionDocTest$DatabaseConnectionPool$Id"] }

I assume it's some breaking change in paradox that crept in, but in lieu of actually tracking that down, giving up syntax coloring in exchange for having something copy-pasteable seems like the next best thing for now.

[leviramsey]

(submitted this from the web editor, so haven't run paradox to verify)

[johanandren]

Still broken with this change, needs a trick like this:

Java
:   @@@ vars
```ruby
akka.actor.typed {
  extensions = ["jdocs.akka.extensions.ExtensionDocTest$DatabaseConnectionPool$Id"]
}
```
@@@