add examples for square roots and general roots

[pkra]

Action item from last TF meeting. Assigned to @dani31415.

[pkra]

@arnog had found a very nifty example on SO https://codepen.io/pkra/pen/YLERdL

[pkra]

Here's a variation I built a while back https://codepen.io/pkra/pen/deJqwQ