[C#] Documentation on how to coordinate the external Onnxruntime-Extension

Is your feature request related to a problem? Please describe.
Onnxruntime refers to the external repository Onnxruntime-Extension which is 9 month old. It is unclear what benefits to include the latest master branch of Onnxruntime-Extension. It is unclear for a given issue using Onnxruntime-extension in c#, which repository, onnxruntime or onnxruntime-extension to file an issue.

There are no samples or test for c# in onnxruntime-extension repo. Most of them are in c++

There is one test for c# related to onnxruntime-extension in onnxruntime csharp folder.

System information

• ONNX Runtime version (latest):

Describe the solution you'd like

We need an improvement supporting C# Custom Op development experience.

[First]
We need to address these feature requests

• 1: Basic build instruction to generate custom_op_library.dll
• 2: Provide custom_op_library.dll

to support better c# for onnxruntime-extension development experience.

[Second]
We need documentation and if possible c# project which combines c++ custom opt dll generation and the c# test project to test-driven the generated custom opt dll.

Start with the existing C# test in the onnxruntime C# folder and the linked build and library issues #11654 and #11653. Review how the onnxruntime and onnxruntime-extension repositories currently coordinate custom-op development. Done means providing build and ownership guidance, plus a documented or demonstrated C++ custom-op DLL and C# test workflow.