(WIP) Add chapter about workflow messages, templates, tokens

This is a developmental draft. It loosely corresponds to the branch https://github.com/totten/civicrm-core/tree/master-msgtpl-ui although there may be back-and-forth revisions needed.

Previews:

• https://think.hm/docs/dev/framework/message/
• https://think.hm/docs/dev/framework/token/