add relplot to lineplot documentation

I think it worth adding an example of how to use relplot to create multi subplots of lineplot to the documentation page of lineplot:
https://seaborn.pydata.org/generated/seaborn.lineplot.html?highlight=lineplot#seaborn.lineplot

as catplot is in the documentation of barplot:
https://seaborn.pydata.org/generated/seaborn.barplot.html?highlight=barplot#seaborn.barplot

