The first question we have to ask is “are you an engineer?” If the answer is yes, then the documentation question is answered - complete, step-by-step annotation of the plans, pictures with every rivet, a time-stamped log so you can track involvement to the nearest minute (including study, shopping and dreaming time). You’ll need backup copies of logs and photos of course, archived every evening...and maybe one done automatically at noon, just to be sure. I know all this becasue I’m an engineer....
From a legal perspective, you will be signing a piece of paper (when you have the airplane inspected) that states (under penalty of perjury) that you built it, and that has always been good enough for the FAA and DAR’s I have used. If you want the Repairman’s Certificate, you will need to prove to an inspector that you were sufficiently involved in teh build to know how to inspect it - most times, that requires photos of you and the airplane in various states of completion (the airplane....not you...). Other than that, records really aren’t required.
Practically speaking, do what feels right for you. My first build was significantly over-documented. Teh second, less so. The third, I took some pictures, and made date annotations in the builder’s manual. My fourth.... well, I refer you to various blogs on teh Kitplanes web site.....and so on, and so on....
Whatever you do - if its a burden that makes you not want to build - re-evaluate!
Paul