Mark System.Drawing.Common as Required in ObjectDetection Example

I was following along with this tutorial and got to the end: https://learn.microsoft.com/en-us/dotnet/machine-learning/tutorials/object-detection-onnx#create-helper-functions

and could not compile because Image was not a recognized class. In previous versions, System.Drawing.Common was a transitive dependency and would get installed. It's not now and so the documentation needs updated to require installing this package from NuGet in the Pre-requisites section: https://learn.microsoft.com/en-us/dotnet/machine-learning/tutorials/object-detection-onnx#create-helper-functions

See here as well: https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/6.0/system-drawing-common-windows-only

Open the Object Detection ONNX tutorial and review the Pre-requisites section near the create-helper-functions anchor. Confirm how the sample uses Image and System.Drawing.Common, then document the required NuGet package there. Done means a reader following the tutorial can install the prerequisite and compile the example.

Written by the indexing model from the issue text.