Consider for a fixed value of $x$ which ca n varies from $x = 0$ to $x = 1$. In between $0$ and $1$, the courses form a triangle with height and base length calculated as below.
$$z = xy$$ $$y + z = 1 - x$$
Solving this equations we have
$$y = \frac{1 - x}{1 + x}$$ $$z = \frac{x(1 - x)}{1 + x}$$
The height of the above triangle is given by the $y$ value above and the base is the y intercept given by $(1 - x)$.
Hence the volume is given by
$$\int_{0}^{1} \frac{1}{2}\frac{x(1 - x)}{1 + x}(1 - x) dx = \frac{17}{12} - 2ln2$$