Hey @vjudys!
I can’t say for sure but it looks like either there WAS a GAS quickstart guide and they removed it and it broke the link, or they intended for there to be one and it was never created as I can’t find one. Looking at the 5.4 documentation, that suggestion is no longer part of the document. There are a ton of tutorials you can use to get started in the learning section, though!
Hope that helps and have fun! The GAS is great!