[Upgrade] Add an api_changes.md: rules for changing protos and CRDs

As discussed across recent upgrade discussions and issues (e.g. [upgrade proposal](https://docs.google.com/document/d/1JduAyGZFyqdNp4UhKiv0EN-is3iW5tf5BwGTWbt2ouI/edit?usp=sharing&resourcekey=0-MXG12QCkleIxhY7cOB7g6A), #954), we currently lack unified guidelines for **evolving** our APIs, Protobuf schemas, and CRDs.

Without a standard document, questions frequently arise regarding:
- How server should handle unknown or unrecognized fields across version upgrades.
- Expectations and review criteria for developers introducing behavioral or schema changes.

I think we should create an `api-changes.md` like kubernetes to talk about API compatibility and provide clear guidance for contributor.