docs: document model provider headers

[adilhafeez]

Summary

• document the optional headers map for model_providers in the configuration reference
• note that legacy llm_providers accepts the same provider fields
• add a concrete headers example to the full config reference and refresh the rendered include

Fixes #948.

Testing

• PATH="/tmp/plano-python-shim:$PATH" PYTHONPATH="$PWD/cli" bash config/validate_plano_config.sh
• git diff --check