Re-organize docs on tokens

Created by: seancolsen

We have this page in the Dev Guide: https://docs.civicrm.org/dev/en/latest/framework/civimail/tokens/

but I kind of think it belongs in the User Guide here: https://docs.civicrm.org/user/en/latest/email/using-tokens-in-emails/

Anyone else want to weigh in?