The Discourse developers themselves thought that a forum user guide was unnecessary, and that the best way to learn to use Discourse forums was via trial and error.
Everyone who creates a Discourse forum account receives a PM with a list of Tips etc …
The “adminbase” (people who use Discourse to run there own forum sites,) kept asking for a User Guide, so one of them started a community project. It was announced over on the Discourse Meta (likely a GitHub project.)
https://meta.discourse.org/t/a-community-effort-to-improve-discourses-documentation/29129
Discourse also has a "learning bot named @discobot …