Does somebody know how to modify the default initial page of the main.cpp when we open the IDE?
Create a the default project you want. Then go to File->Save project as template... Now when you want to create a new one just go to File->New->From template...
I just realized there is actually a better option for what you wanted to do.
If you go to Settings->Editor...->On the left side there is a tab that reads "Default Code" Then you can enter the code you want and hit Ok.
Yes, but that option is not for setting the main.cpp, but the header or source files (in general)