Contributor guide doesn't reflect refactoring in UDF

[edmondop]

Describe the bug

With https://github.com/apache/datafusion/issues/8708 we have simplified AggregateFunctions, however we haven't updated the documentation in all necessary places

To Reproduce

The how-to guide describes a way of adding functions that is not valid anymore, see nhttps://github.com/apache/datafusion/blob/main/docs/source/contributor-guide/howtos.md?plain=1

Expected behavior

No response

Additional context

No response