doc: fix external links with 404 status

PR-URL: https://github.com/nodejs/node/pull/15463
Fixes: https://github.com/nodejs/node/issues/15462


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>