Library Guide: Adding User Defined Functions: Scalar/Window/Aggregate/ #7307

Is your feature request related to a problem or challenge?

Part of #7014

If we want to have DataFusion used as the core of many new systems, we need it to be as easy as possible for someone to get their idea working on top of DataFusion.

Thanks to @tshauck we now have a basic Library Users Guide ❤️ and this ticket describes expanding it out

Describe the solution you'd like

Fill in the content of https://arrow.apache.org/datafusion/library-user-guide/adding-udfs.html

Example Outline

1. How to create and register a ScalarUDF
2. How to create and register a AggregateUDF
3. How to create and register a WindowUDF

The examples directory holds a bunch more of examples: https://github.com/apache/arrow-datafusion/tree/main/datafusion-examples

Describe alternatives you've considered

No response

Additional context

No response