Update ChangeLog and README for child-pipelines restructure

Updates documentation to reflect the new nested structure for child-pipelines options in 0.18.0:

• Manual entries: child-pipelines.flakes.* (instead of child-pipelines.*)
• Auto-discover: child-pipelines.auto-discover.enable (instead of child-pipelines-auto-discover.enable)

This aligns the docs with the implementation on the bump-0.17.0 branch.