gh-129699: Add description to IDLE doc title (#129727)

Also extend the 'idlelib' section header. These additions affect both the displayed idle.html file and the contents.html file displayed by clicking the Complete table of contents link on the main docs.python.org page. (The module index entries are generated from the module name and synopsis within module files.)
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Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>