Docs: hello world pod

[sdmoralesma]

It might be useful for newcomers to know how to create simple and runnable Pod. Additionally some advice on when to create a native Pod might be useful, for example, from the slack thread:

Some pods are written in other languages, because low level stuff like file system events are better done in golang or Rust. Some things simply didn't work with GraalVM, like sqlite, so that's why its in golang.

One dev tip: you can test the pod first on the JVM using the pod library and/or starting it via ["clojure" "-M" "-m" ...]
and if it works, then you can try compiling it to native