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.
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