Hello world with Buf examples, starting from Java

At the moment, the documentation contains a non-trivial example for Kotlin, which is a little counterintuitive. I propose the documentation offer "hello world"-style examples that only focus on the specific of setting up Connect in a specific language, leaving more advanced features and use cases in a separate section of the doc.

Also, since Java is still a popular language, and Kotlin is Java compatible, probably it would be great to have a "Getting started with Java" that mentions that the code generated is in Kotlin, and you need a certain setup, but you can use Connect when coding in Java

Reference: https://connectrpc.com/docs/kotlin/getting-started/

Start with the referenced Kotlin getting-started documentation at https://connectrpc.com/docs/kotlin/getting-started/. Separate the basic Connect setup from advanced examples and add a Java getting-started path that explains the Kotlin-generated code and required setup; done means both languages have focused Hello World guidance.