test: test and docs for detached fork process

This tests child process fork component in detached mode
by spawning a parent process that creates a child process.
We kill the parent process and check if the child is still
running.

Fixes: https://github.com/nodejs/node/issues/17592

PR-URL: https://github.com/nodejs/node/pull/24524


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>