doc: move StackOverflow to unofficial section

Fixes: https://github.com/nodejs/node/issues/19412

PR-URL: https://github.com/nodejs/node/pull/19416


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>