Document fin flutter example

The fin flutter example is currently undocumented. It'd be great if it could be added to the examples page in the HTML documentation like the [lid driven cavity flow](http://cusfsim.readthedocs.org/en/latest/examples.html#lid-driven-cavity-flow) example.

This requires creating the documentation itself in, e.g., examples/plot_fin_flutter.rst and then adding an entry for that in doc/examples.rst.