Add compatibility guidelines to API documentation

Description

When working on the API, there is no documentation describing a practice, or strategy for backwards compatibility. We should add a section describing how to handle breaking changes. Maybe answer a few questions like:

• When should you ensure backwards compatibility?
• How to track old code/features/methods that can be removed after major versions
• Formalities for upgrading to API versions with breaking changes

Proposal

Add a section or completely new document in https://docs.gitlab.com/ce/api/