bpo-35087: Update idlelib help files for the current doc build. (GH-10162)

There is only one trivial change to idle.rst. Nearly all the changes to help.html are the elimination of chapter and section numbers on headers due to changes in the build system. help.py no longer requires header numbering.
(cherry picked from commit db40cb5)

Co-authored-by: Terry Jan Reedy <[email protected]>

Author: miss-islington