bug#47099: Add example of gnus-part-display-hook

[積丹尼 Dan Jacobson]

(info "(gnus) Customizing Articles")
says
      You can, of course, write your own functions to be called from
   ‘gnus-part-display-hook’.  The functions are called narrowed to the
   part, and you can do anything you like, pretty much.  There is no
   information that you have to keep in the buffer—you can change
   everything.

OK, but please add at least one example.