[3.13] gh-139707: Copy-strip change to idle.rst into idlelib

Copy net new text in idle.rst from main to 3.13.
Make 3.13 docs and copy-strip idle.html to idlelib/help.html.
See #139707 (comment)
and PR #143718 message for more.

Author: terryjreedy

Doc/library/idle.rst

@@ -37,6 +37,11 @@ IDLE has the following features:
 
 * configuration, browsers, and other dialogs
 
+The IDLE application is implemented in the :mod:`idlelib` package.
+
+This is an optional module. If it is missing from your copy of CPython, look for documentation from your distributor (that is, whoever provided Python to you).
+If you are the distributor, see Requirements for optional modules.
+
 Menus
 -----