Update CI onboarding guidelines

CI onboarding guidelines (https://github.com/dotnet/source-build/blob/main/Documentation/planning/arcade-powered-source-build/onboarding/ci-onboarding.md) are out-of-date and do not provide relevant examples.

For instance, `sourceBuildParameters` section is outdated.

Please update and any related guidance in Arcade, i.e. in YML comments, if needed.