Add 'plot element set' to examples or visualization section

Show how to plot element sets, either in the examples or in the visualization how-to. Something like

```python
plotter = pv.Plotter()
plotter.add_mesh(model.mesh.to_pyvista(), color="white", opacity=0.5)
plotter.add_mesh(element_set.mesh.to_pyvista(), color="green")
plotter.show()
```