Add section about using `pipe` to the user guide

[KDruzhkin]

Add section about using `pipe` to the user guide

[stinodego]

It makes sense to add a dedicated section for pipe in the docs, as it's a useful tool for doing custom things. It happens more often that I point out "you can do this with pipe" and people didn't know that method existed.

Not sure I would call it 'debugging with pipe' though. It's useful for more than just debugging.

A "Pipe" page could be added to the "Transformations" section. This could showcase common patterns for using pipe. The schema assertion could be one, but a more important pattern is when you want to "name" some custom transformations and use them in a chaining fashion. We can include examples for Expr, DataFrame, and LazyFrame.