Add chapter about workflow messages. Update token docs.

Resubmission of !962 (closed), which documents a number of updates circa 5.43.

Ping @eileen

Two major areas:

(1) Add chapter on workflow messages

Preview: https://think.hm/docs/wfmsg/framework/message/

(2) Update chapter on tokens

Preview: https://think.hm/docs/wfmsg/framework/token/

TOC Before:

TOC After: