doc: add inspector usage example

Add a simple example showing how to use the inspector API to access
the CPU profiler.

PR-URL: https://github.com/nodejs/node/pull/19172


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>