bpo-34451: Document prompt and output toggle feature in html tutorial (GH-27105)

Author: thmsdnnr

Doc/tutorial/introduction.rst

@@ -11,6 +11,13 @@ with a prompt are output from the interpreter. Note that a secondary prompt on a
 line by itself in an example means you must type a blank line; this is used to
 end a multi-line command.
 
+.. only:: html
+
+   You can toggle the display of prompts and output by clicking on ``>>>``
+   in the upper-right corner of an example box.  If you hide the prompts
+   and output for an example, then you can easily copy and paste the input
+   lines into your interpreter.
+
 .. index:: single: # (hash); comment
 
 Many of the examples in this manual, even those entered at the interactive