Move code in `user_defined_plan.rs` to the `extending-operators` doc

[xudong963]

From the intention: https://github.com/apache/datafusion/blob/main/datafusion/core/tests/user_defined/user_defined_plan.rs#L18-L24, I think it's better to put the code in examples.

Also, the code isn't solid, and its tests are fragile, such as #15744 (comment).

cc @alamb

[alamb]

I agree -- better still would be to put the content into https://datafusion.apache.org/library-user-guide/extending-operators.html

[Adez017]

HI @xudong963 , i want to work on this , but could you clarify what exactly we need to do ?

[xudong963]

@Adez017 Hi, the file contains an example about how to define an extending operator, what we need to do is to move the case to the https://datafusion.apache.org/library-user-guide/extending-operators.html doc. You can refer the custom table provider doc format: https://datafusion.apache.org/library-user-guide/custom-table-providers.html