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>

doc/api/addons.md

@@ -221,7 +221,7 @@ illustration of how it can be used.
 > Stability: 1 - Experimental
 
 N-API is an API for building native Addons. It is independent from
-the underlying JavaScript runtime (ex V8) and is maintained as part of
+the underlying JavaScript runtime (e.g., V8) and is maintained as part of
 Node.js itself. This API will be Application Binary Interface (ABI) stable
 across version of Node.js. It is intended to insulate Addons from
 changes in the underlying JavaScript engine and allow modules
@@ -232,6 +232,41 @@ set of APIs that are used by the native code. Instead of using the V8
 or [Native Abstractions for Node.js][] APIs, the functions available
 in the N-API are used.
 
+To use N-API in the above "Hello world" example, replace the content of
+`hello.cc` with the following. All other instructions remain the same.
+
+```cpp
+// hello.cc using N-API
+#include <node_api.h>
+
+namespace demo {
+
+napi_value Method(napi_env env, napi_callback_info args) {
+  napi_value greeting;
+  napi_status status;
+
+  status = napi_create_string_utf8(env, "hello", 6, &greeting);
+  if (status != napi_ok) return nullptr;
+  return greeting;
+}
+
+napi_value init(napi_env env, napi_value exports) {
+  napi_status status;
+  napi_value fn;
+
+  status = napi_create_function(env, nullptr, 0, Method, nullptr, &fn);
+  if (status != napi_ok) return nullptr;
+
+  status = napi_set_named_property(env, exports, "hello", fn);
+  if (status != napi_ok) return nullptr;
+  return exports;
+}
+
+NAPI_MODULE(NODE_GYP_MODULE_NAME, init)
+
+}  // namespace demo
+```
+
 The functions available and how to use them are documented in the
 section titled [C/C++ Addons - N-API](n-api.html).