Improved documentation

• The example notebook lacks documentation (but not much!). A small text that explains what are the beams different beams, the grey area, and so on would be useful.

• It would be great to show a legend for the plots describing which beam is which. If I get this right, the low energy and high energy cases are both overlayed for each beam. The grey are the 0th order beam? The legend can be outside of the plot to avoid saturating it.

• Grating: since the zone plate has a grating element and there is also the angle of the MONO grating, I got confused which grating this refers to, but now I get it that this can only be the TZPG grating. One word or two in the documentation would be great.