Write and present migration documentation for compiler warning policy migration

[nradakovic]

What

Ensure downstream users understand what changes, what remains compatible, and what actions are required on their side.

Acceptance Criteria (DoD)

• Migration documentation is written and reviewed.
• The documentation explains how to move from toolchain-owned warning configuration to policy-owned warning configuration.
• The relevant user groups of the C/C++ toolchains have been informed.
• Feedback from those groups has been collected and tracked where needed.

How

• Write migration guidance for existing users of the C/C++ toolchains.
• Describe the old model and the new model.
• Call out required changes, compatibility expectations, and examples.
• Present the migration plan to the different groups using the C/C++ toolchains.
• Gather feedback and capture follow-up actions if issues are identified.