Improved feature explanations and code runners #65

Add new and improve existing explanations of impactful C++ features, available when pressing on a feature's info button.

Each feature gets three sections: what's new, why it matters, and an example code snippet if necessary.

Each code example should be immediately executable and should also be editable so that the user is allowed to explore the feature.

This should make cppstat a better resource for beginners as well as people that are curious why certain features have been added to C++, and how they work.

The explanations of major C features should be improved as well, following the same format.

Sample: