Add comprehensive documentation for offline build troubleshooting and Microsoft.Build.NoTargets SDK resolution

[Copilot]

When building .NET 10.0.0-preview.6 and later in offline environments (such as sandboxed builds), users encounter SDK resolution failures that prevent prep-source-build.sh from executing successfully:

/build/dotnet-10.0.0-preview.6.25358.103/eng/init-detect-binaries.proj : error : Could not resolve SDK "Microsoft.Build.NoTargets"
/build/dotnet-10.0.0-preview.6.25358.103/eng/init-detect-binaries.proj : error : SDK resolver "Microsoft.DotNet.MSBuildWorkloadSdkResolver" returned null
/build/dotnet-10.0.0-preview.6.25358.103/eng/init-detect-binaries.proj : error MSB4236: The SDK 'Microsoft.Build.NoTargets' specified could not be found.

The root cause is that init-detect-binaries.proj requires the Microsoft.Build.NoTargets SDK to execute, but offline builds cannot download it from online NuGet feeds. While the binary detection tooling itself is configured to use offline restore sources via --with-packages, the project file needs to resolve its SDK dependencies before it can execute.

This PR provides comprehensive documentation to address the issue:

New Documentation

• troubleshooting-offline-builds.md - Complete troubleshooting guide covering the problem, root cause, and practical workarounds
• vmr-sdk-configuration-guidance.md - Technical implementation guidance for VMR developers to implement the permanent solution

Immediate Workarounds Documented

1. NuGet Configuration Method: Add both the packages directory and SourceBuildReferencePackages subdirectory to NuGet.config
2. Stage 2 Build Workaround: Use -p:SkipArcadeSdkImport=true when binary detection isn't critical
3. Proper Offline Source Setup: Guidelines for configuring offline package sources correctly

Documentation Updates

• Updated README.md to reference the new troubleshooting guide
• Enhanced bootstrapping-guidelines.md and how-to-stage2-build.md with offline scenario guidance
• All documentation follows repository markdown standards with proper cross-references

Implementation Roadmap

The documentation provides clear technical guidance for implementing the permanent solution in the VMR repository, which involves factoring SDK configuration logic from build.sh and init-source-only.proj to be shared with prep-source-build.sh for offline builds.

This ensures users have immediate solutions while providing a clear path for the VMR team to implement the complete fix.

Fixes #5286.

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