An example about contour plot

hi, I write an example about contour plot and animation when l learned how to use Stipple. I met some problems and I solved them. I think this example may help people understand Stipple better. So, I hope this example can be added to StippleDemos. If allowed, I will start a pull request.

This is the example:

Review the StippleDemos repository structure and the linked contour-plot animation first; the issue names no target file, source example, or test. Done means adding a runnable contour-plot animation demo in the repository and documenting how to use it, but the expected implementation details are not specified.

Written by the indexing model from the issue text.