score:6
There is no official list of supporting documentation required for applying for a US visa. As you've discovered, there are lists of 'recommended' documents, however these are normally written for applicants who are citizens of countries that are not eligible for the Visa Waiver Program, where there is deemed to be a higher likelihood of passengers who will overstay their visa. (Overstay rate is actually one of the criteria for a country to be considered for entry to the Visa Waiver Program!).
Your situation is obviously a little different, as you seemingly are from a VWP country but not eligible for entry under the VWP due to your trip to North Korea.
Whilst taking at least some of the documentation suggested may still be a good idea, the more important documentation to take to your interview will be details of your trip to North Korea which show that it was indeed done as a tourist and nothing more. This might include booking details with the company you did the trip with (presuming it was an organized tour as such trips generally are), an itinerary, etc. You've stated that the trip was 'obviously done for touristic reasons', however the simple fact is that there's nothing 'obvious' about this for the consulate staff - they will need to be convinced that it was indeed a tourist trip, and thus that allowing you into the US will not pose a risk for security.
Additional documentation such as invitation letters from your friend in the US is almost certainly not required, however it would still be advisable to take proof of funds and proof of a stable job in your home country - it's very possible these will not be asked for, but if they are and you don't have them then it will at least delay your application.
Obviously it's not possible to give a definitive answer to a question like this, as exactly what is needed will depend on factors that we don't know. The more evidence you can take the lower the chances of rejection/delays, however it's unlikely that it will be needed.