Add n-day moving window example plot

I added an example to the streamflow_duration_hydrograph_examples.rst file to show how to use the 'leading' and 'trailing' arguments to construct a 30-day moving window from which to calculate streamflow percentiles for each day of the year, and then plot those percentiles with a year of streamflow values (addresses #13 (closed)). I walked through the example and I think I understand why the plot is displaying as such, but I would love some feedback from @shamshaw on whether this is typically what a moving-window percentile plot looks like (kind of blocky) and if the narrative is clear.