@sdetweil
There is documentation on how templates work in the benchpress repository - that is the templating engine NodeBB uses: https://github.com/benchpressjs/benchpressjs/tree/master/docs
There is a link to the engine in getting started development docs (https://docs.nodebb.org/development/) but I admit that it's not intuitive.
The documentation for working with included helpers and client-side modules is basically nonexistent though, I mostly just look at other plugins (quickstart is a good reference for common use cases) and ultimately just look at the code. For the included setting types you can see it here: https://github.com/NodeBB/NodeBB/tree/master/public/src/modules/settings