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

[Henk-Jaap Wagenaar]

Changes by Henk-Jaap Wagenaar <wagenaarhenkjaap at gmail.com>:


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nosy: +Henk-Jaap Wagenaar

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