gh-75500: Add idlelib section to IDLE doc (GH-95832)

This enables accessing IDLE as 'idlelib' in the Doc Module listing.
(cherry picked from commit 70fc964)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Author: miss-islington

Doc/library/idle.rst

@@ -983,3 +983,20 @@ changed with the Extensions tab of the preferences dialog. See the
 beginning of config-extensions.def in the idlelib directory for further
 information.  The only current default extension is zzdummy, an example
 also used for testing.
+
+idlelib
+^^^^^^^
+
+.. module:: idlelib
+   :synopsis: Implementation package for the IDLE shell/editor.
+
+**Source code:** :source:`Lib/idlelib`
+
+The Lib/idlelib package implements the IDLE application.  See the top
+of this file or content listing on the left for how to use IDLE.
+
+The files in idlelib are described in idlelib/README.txt.  Access it
+either in idlelib or click Help => About IDLE on the IDLE menu.  This
+file also maps IDLE menu items to the code that implements the item.
+Except for files listed under 'Startup', the idlelib code is 'private' in
+sense that feature changes can be backported (see :pep:`434`).

Lib/idlelib/help.html

@@ -113,6 +113,7 @@ <h3><a href="../contents.html">Table of Contents</a></h3>
 <li><a class="reference internal" href="#setting-preferences">Setting preferences</a></li>
 <li><a class="reference internal" href="#idle-on-macos">IDLE on macOS</a></li>
 <li><a class="reference internal" href="#extensions">Extensions</a></li>
+<li><a class="reference internal" href="#module-idlelib">idlelib</a></li>
 </ul>
 </li>
 </ul>
@@ -1001,6 +1002,17 @@ <h3>Extensions<a class="headerlink" href="#extensions" title="Permalink to this
 information.  The only current default extension is zzdummy, an example
 also used for testing.</p>
 </section>
+<section id="module-idlelib">
+<span id="idlelib"></span><h3>idlelib<a class="headerlink" href="#module-idlelib" title="Permalink to this heading">¶</a></h3>
+<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/main/Lib/idlelib">Lib/idlelib</a></p>
+<p>The Lib/idlelib package implements the IDLE application.  See the top
+of this file or content listing on the left for how to use IDLE.</p>
+<p>The files in idlelib are described in idlelib/README.txt.  Access it
+either in idlelib or click Help =&gt; About IDLE on the IDLE menu.  This
+file also maps IDLE menu items to the code that implements the item.
+Except for files listed under ‘Startup’, the idlelib code is ‘private’ in
+sense that feature changes can be backported (see <span class="target" id="index-7"></span><a class="pep reference external" href="https://peps.python.org/pep-0434/"><strong>PEP 434</strong></a>).</p>
+</section>
 </section>
 </section>
 
@@ -1054,6 +1066,7 @@ <h3><a href="../contents.html">Table of Contents</a></h3>
 <li><a class="reference internal" href="#setting-preferences">Setting preferences</a></li>
 <li><a class="reference internal" href="#idle-on-macos">IDLE on macOS</a></li>
 <li><a class="reference internal" href="#extensions">Extensions</a></li>
+<li><a class="reference internal" href="#module-idlelib">idlelib</a></li>
 </ul>
 </li>
 </ul>
@@ -1153,7 +1166,7 @@ <h3>Navigation</h3>
 <br />
     <br />
 
-    Last updated on Aug 05, 2022.
+    Last updated on Aug 07, 2022.
     <a href="/bugs.html">Found a bug</a>?
     <br />