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.
This needs to be

    ```notrust
    (area of unit circle) / (area of square)
    ```

because rustdoc defaults to running code blocks.