doc: update WASM code sample

- Code sample updated by adding a hello-world (`demo.wat`) code example
- Step for compiling `.wat` to `.wasm` added (with reference to `wabt`
  tools)
- The sample code prints "hello world\n" in the console

This update adds a very minimal change to the existing sample and can be
treated as an extension.

PR-URL: https://github.com/nodejs/node/pull/33626


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>