document how to use ExcelReport in tutorial

[gdementen]

• we shouldn't pass too much time about ExcelReport itself because the API might disappear entirely after #900, but we should at least mention it exists, explain what it's useful for and add a screenshot for the result.
• On the other hand, a section about how to list existing Excel templates, where to store them, create new ones, etc. is sorely needed (I am unsure myself how to do this) and will still be useful whatever we do about the API