Update QuickStart.rst with demo scripts

[wilson-jetbloom]

[pekkaklarck]

Splitting the Execution section into two is a good idea. I don't like showing only how to clone the project with git, though, because it is not certain that everyone interested on this will know anything about git or have it installed. We could show how to download the guide using git in addition to mentioning that it can be downloaded also otherwise, but I expect everyone with any git experience knowing git clone already. Also, section titles currently have only the first word in caps, not all words in the title.