Hello Sir,
Though I did not check the documents, but when I started with the first document, it told me to go to – https://en.m.wikipedia.org/wiki/The_unanswered_questions
So I went there and it says that Buddha did not answer this question that how does a being come into existence. But if Buddha has told so many stories about this world, that world, Gods, Brahmas, humans, their many lives, he also has clearly talked at lengths about the end of cycle of birth and death then why not the start. This is an issue of mystery and dissatisfaction.