I actually find the overview that you linked to not particularly useful when I first started. In fact, I looked at it, got scared by it, and went back to 3.3.7 for another month. Having said that, and after trying various resources, I find that the best way for me is to learn the framework is still the Bootstrap documentation (this link explains the grid system and is, in my opinion, the actual starting point in the documentation for beginners).
Once your foundation is solid with using the grid system, quickly scroll through to see what components are available (for recalling later on) and simply refer back to the documentation when you need something done. Beyond the grid system, common components such as buttons and forms, and some simple formatting classes, I personally think that there is really no need to remember anything off by heart to begin with.
I hope that helps.