doc: add note about ssh key to releases

PR-URL: https://github.com/nodejs/node/pull/31856


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>