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

先閱讀 cmd.Cmd 文件和 issue #143804，以了解現有的方法級說明以及回報的困惑。新增一個使用層級的教學，介紹 emptyline() 和 do_EOF，包括預期行為和乾淨退出，然後確認文件是否清楚地說明這兩個覆寫方法。