civimail.md - Move general token-handling docs to a separate file

These documents are misleading -- they talk about CiviMail because, *historically*,
much of this grew out of CiviMail. However, *today*, tokens are used more generally.
Also, in `civicrm-core`, `TokenProcessor` is *not* used by CiviMail. (You have to
install FlexMailer to use it.)