Step by step hello world

This MR fixes a few minor issues and refactors the getting started tutorial to be less overwhelming for beginners (it was for me). This is based on a few assumptions:

• Starting small and slowly introducing new elements rather than immediately presenting the content and then going over it is less overwhelming for beginners.
• Starting with the section of code that provides more concrete/visual results sticks to the user.
• Starting with a minimal example showcases how much you can do in a few lines, and is satisfying.
• Simple explanations for minor things to avoid accessing third party resources (like knowing that QString() default initializes to null or that it's generally good to set unchangeable strings as read only) is less distracting as the reader doesn't need to leave the website temporarily.
• Keywords should be explicitly stated to make it easier for learners to search on their own (like mentioning ternary operators for beginner/intermediate C++ students who might never have seen it before despite being perfectly capable of understanding the rest).

I think going from KGuiItem to KAboutData/KI18n to QCommandLineParser works better for that purpose.

Please review if the content is correct as well, I am still new at this.