Add example of estimating pi using Monte Carlo simulation to std::rand

Pros:
I like this example because it's concise without being trivial. The Monty Hall example code is somewhat lengthy and possibly inaccessible to those unfamiliar with probability.

Cons:
The Monty Hall example already exists. Do we need another example? Also, this is probably inaccessible to people who don't know basic geometry.
Also, this could be phrased as just

> Since the area of the unit circle is &pi;, we have
>
>     (area of unit circle) / (area of square) = π/4
>
> So if ...