Extend onboarding by more explicit information

.gitlab/issue_templates/Onboarding.md: Make the required information in the Details section more explicit and add entries that are relevant when creating an SSO and/or archweb account. Add a note for sponsors of new users, so that they also add a clearsigned version of the data they provide. Add a dot at the end of each sentence. Make the entries for mailing list operations more generic and rely on the communication e-mail address, which may be the user's personal mail address or a newly created @archlinux.org mail address.