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.

Read Documentation/Style Guide/Code Examples and the doctest documentation, especially the interactive examples and directive guidance. Review the Docs/Doctest CI check and the referenced discussions to establish the current policy, then add a concise summary in the existing section or a new section immediately after. Done means the guide explains expected doctest behavior and relevant directives for interactive examples.