[docs] [issue31421] IDLE doc: add section on developing tkinter apps.

[Terry J. Reedy]

Changes by Terry J. Reedy <tjreedy at udel.edu>:


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keywords: +patch
pull_requests: +3508
stage: needs patch -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31421>
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