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

先阅读 cmd.Cmd 文档和 issue #143804，以了解现有的方法级说明以及报告中的困惑。添加一个使用级教程，介绍 emptyline() 和 do_EOF，包括预期行为和干净退出，然后验证文档是否清楚地解释了这两个重写方法。