Update `score_cpp_policies` documentation for compiler warning features

[nradakovic]

What

Make the warning policy discoverable and understandable for users adopting score_cpp_policies. Document every feature with which flags enables and why this feature belongs to this group.

Acceptance Criteria (DoD)

• score_cpp_policies documentation includes compiler warning feature documentation.
• Users can understand which warning features exist and how to enable them.
• Documentation explains the intended relationship between the toolchain module and the policy module.
• Examples are included where needed to support adoption.

How

• Document the available compiler warning features.
• Explain how consumers enable warning levels and warnings-as-errors behavior.
• Clarify the ownership boundary between toolchain implementation and policy definition.
• Add examples where helpful.