Document ImageJ Measure comparison

[yangfei]

Summary

• Add the first ImageJ Measure comparison document under docs/.
• Record ImageJ 1.54s99, test image, rectangle ROI coordinates, and compared values.
• Document that Area, Mean, Min, and Max match for the tested cases.
• Document the StdDev difference and link follow-up issue Align Measure standard deviation with ImageJ #57.

Verification

• dotnet build ImageJCsharp.sln
• dotnet test tests/ImageJCsharp.Core.Tests/ImageJCsharp.Core.Tests.csproj
• dotnet test tests/ImageJCsharp.App.Tests/ImageJCsharp.App.Tests.csproj

Closes #6