Exposing more project settings for documentation

[AThousandShips]

For project settings to be available for documentation, more importantly for doctool and ci to work, they have to be defined before this line: https://github.com/godotengine/godot/blob/d8e242cba8fe33f2d4e3c3e4d5f08038ab063888/main/main.cpp#L2525

Moved a number of settings from below this to project_settings.cpp, could also move them inside main.cpp but felt this was a better place to put them, can also move other settings out of main but left ones not causing this problem to keep the changes minimal

Documented a few of the settings that I had a clear idea of what they did and how to explain.