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.
`sqrt` is *technically* unnecessary here. In my experience it's generally poor form to compare distances rather than squared-distances (for obvious performance reasons), and I think we want to generally encourage idiomatic behavior in examples, but its presence might have a positive effect on overall clarity. Just a thought.