doc: add AsyncResource + Worker pool example

Use Worker thread pools as an example of how `AsyncResource`
can be used to track async state across callbacks.

PR-URL: https://github.com/nodejs/node/pull/31601


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>