Add definition of "breaking changes"

What does this MR do?

The documentation refers to "breaking changes" in multiple places, but nowhere defines the term. This introduces a definition in the contribution guide, and adds links to this definition in multiple places.

Notably, the GitLab Handbook also refers to "breaking changes" in multiple places without defining it. If merged, this could serve as the SSOT for the Handbook as well.

The definition in this MR was inspired by Oliver Matthews, who proposed: "a change that will require other people to make a corresponding change."

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team