Add private gallery page layout contracts

[PrzemyslawKlys]

Summary

• add indexLayout, moduleLayout, and documentLayout support to portal-module-pages
• emit meta.pageKind and page-specific counts/metadata for generated catalog, module, and related-document pages
• document the reusable layout contract and schema options

Validation

• dotnet test .\PowerForge.Tests\PowerForge.Tests.csproj -c Release --filter PortalModulePagesTests --no-restore
• dotnet test .\PowerForge.Tests\PowerForge.Tests.csproj -c Release --filter "PortalModulePagesTests|PortalDocsIndexTests" --no-restore --logger "console;verbosity=normal"

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