Make documentation example closer to domain

[alex-senger]

Examples in our documentation are very close to programming in python, e.g:

function f(y: Int): Int {
    var x: Int;
    x = y;
    return x;
}

Replace them with examples which are closer to our domain, e.g:

function mehrwertssteuer(base: Float): Float {
    if (...) {
        return base * 1.19
    } else {
        return base * 1.07
    }
}