[FEATURE] Add documents on how to develop a new UDF/UDAF

[yuancu]

Is your feature request related to a problem?

Current frameworks for creating new user-defined functions (UDF) / user-defined aggregation functions (UDAF) is a little bit involved. Developers has to take some time to dive into different parts of the code base to learn how to create a new UDF / UDAF.

What solution would you like?

We should create a document somewhere with examples to instruct developers on how to create a new UDF / UDAF.

What alternatives have you considered?
A clear and concise description of any alternative solutions or features you've considered.

Do you have any additional context?
originally discussed at #4024 (review)