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GH-38594: [Docs][C++][Gandiva] Document how to register Gandiva external functions (#38763)

### Rationale for this change
Provide a basic documentation on how to register and develop Gandiva external functions, including IR functions and C functions.

### What changes are included in this PR?
A markdown doc providing an overview and detailed steps for integrating external functions into Gandiva. 

### Are these changes tested?
It is a doc change

### Are there any user-facing changes?
No
* Closes: #38594

Lead-authored-by: Yue Ni <niyue.com@gmail.com>
Co-authored-by: Yue <niyue.com@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>