doc: add "Hello world" example for N-API

Our Addons documentation has a "Hello world" example that
outlines all steps to build it. Adding the sources for this
"Hello world" example for N-API.

PR-URL: https://github.com/nodejs/node/pull/17425


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>