[Docs][C++][Gandiva] Document how to register external functions #38594
Description
Describe the bug, including details regarding any error messages, version, and platform.
In this PR (#38116), we introduce some new external function registration APIs to allow Gandiva developers to author third party functions and register them so that these external functions can be used in Gandiva expressions.
This entire process requires some documentation so that developers can get started to use this capability:
- which APIs are needed, and the descriptions for the registration API
- what these APIs are capable
- how to generate LLVM IR
- etc
Component(s)
C++ - Gandiva