arrow_demo is not very basic...

As pointed out in https://github.com/matplotlib/matplotlib/pull/20390#issuecomment-857854570, `arrow_demo.py` (https://matplotlib.org/stable/gallery/text_labels_and_annotations/arrow_demo.html) is _very_ specialized and quite complicated for something that claims to be a general arrow demo.  It should be moved or put at the bottom of a more straightforward demo.  