Is your feature request related to a problem? Please describe.

Developing engines needs to know what result fields are supported by the templates.

Describe the solution you'd like

Add a documentation about the HTML templates used in the simple theme / I assume we can focus on the simple theme, compare discussion at #162

Additional context

IMO the result.<field> is the API of the templates and should be documented, otherwise a engine developer has to look at the Jinja templates which are hard to read.