Extend onboarding information for signing keys

.gitlab/issue_templates/Onboarding.md:
Extend the information for onboarding of packager signing keys and new
main signing keys, including comments on who is supposed to create the
tickets towards the archlinux-keyring project.
2 jobs for improve_onboarding_for_keyholders in 1 minute and 1 second (queued for 4 seconds)