Update example.ipynb with plot_pfss and pfss_3d

[jgieseler]

~~Update docstrings for plot_pfss and pfss_3d~~ [DONE], and add them to the API refrence of readthedocs.

Also add their usage to https://github.com/jgieseler/solarmach/blob/main/examples/example.ipynb

Add example with return_plot_object=True and fig.write_image("plot.png") as well as fig.write_html("solar-mach.html", include_plotlyjs='cdn') # and include_plotlyjs=True