Add discussion of doctesting interactive code examples.

There have been various discussions about doctesting interactive code examples in the python docs. The consensus has been that it is desirable that they pass. Indeed, CI currently has a (pass-not-required) Tests / Docs / Doctest. (But it seems to be setup to be excessively lenient, passing many errors that are normally caught. Intended? Bug?) There has been additional discussion about use of particular features, such as doctest directives. An example from the doc:

The [ELLIPSIS](https://docs.python.org/3/library/doctest.html#doctest.ELLIPSIS) directive
gives a nice approach for the last example:
>>> C()  # doctest: +ELLIPSIS
<C object at 0x...>

Other people may remember such discussions better. In any case, a summary of current policy should be added either in
Documentation / Style Guide /Code Examples or in a new "Interactive Code Examples and Doctest" section immediately after.

Documentation/Style Guide/Code Examples と doctest のドキュメント、特に対話的な例とディレクティブに関するガイダンスを読む。Docs/Doctest CI チェックと参照されている議論を確認して現在のポリシーを把握し、その後、既存のセクションまたはその直後の新しいセクションに簡潔な概要を追加する。ガイドで、対話的な例に対して期待される doctest の動作と関連するディレクティブが説明されていれば完了とする。