Add documentation for Cxx API snapshot generation (#56146)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56146

Adds documentation for the cxx-api snapshot generation pipeline, covering usage commands for generating and checking snapshots, and configuration.

It answers:
1. What the script does.
2. What the output looks like.
3. How to use it (generate, check).
4. How it works (Doxygen → parser → .api files).
5. How it can be configured (config.yml).
6. What to do when snapshots differ.

Changelog:
[Internal]

Reviewed By: huntie

Differential Revision: D96316004

fbshipit-source-id: b527606b87efb2af72680b572135a5fc595ab85e