doc: add gpg key export directions to releases doc

Adds an extra step with instructions for exporting a gpg key to be
uploaded to the key server.

PR-URL: https://github.com/nodejs/node/pull/35298


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>