Short summary: in a Japanese journal from 1997, an employee of Ikegami Tsushinki (the company that programmed Donkey Kong for Nintendo) wrote about the process. He still had documentation from back in 1981, and his account provides brand-new historical details.
Critical Kate happened across the journal and, knowing that its length (11 pages) would be beyond the usual remit of volunteer translations, organized a fundraiser with Hit Save! and Gaming Alexandria to raise $750 to commission a translation from Shmuplations. The goal’s already been met, but you can still donate if you’d like!