Add a quickstart page

[wescopeland]

From the index, there should be a giant link that users can press which walks them through making an account, downloading an emulator, and hopping into a (popular) game and playing as quickly as possible.

[PedroHenriqueAtalla]

i have made a page for it! it explains how to create an account, log in, how to download an emulator, how to set up both RetroArch and Bizhawk, and how to set up RetroArch Mobile. everything has been capitalized, there are images, and it's all been properly segmented. it should also not be a long read.

quick-start.md

by the way, in case it isn't good enough or if it doesn't match the standards, no need to accept it okay? i made this just to help, so in case it's improper or if i messed up somewhere, no need to put it in the docs.