Add a tutorial for cmd.Cmd for `emptyline()` and `do_EOF`

Documentation

This is w.r.t the issue #143804

It was pointed out it is not a bug (agreed) but we can improve the documentation of this module
since it is now not very clear that

• One has to over-ride "emptyline" so that the last line is not repeated
• One has to over-ride do_EOF to quit cleanly

A tutorial on this in the documentation would be helpful as right now these are at the level of method level documentation and quite unclear at the usage level.

Linked PRs

• gh-145292

cmd.Cmd のドキュメントと issue #143804 から始めて、既存のメソッドレベルの説明と報告されている混乱を理解します。emptyline() と do_EOF を扱う使用レベルのチュートリアルを追加し、期待される動作と正常な終了を含めます。その後、ドキュメントで両方のオーバーライドが明確に説明されていることを確認します。