Clean up the example projects and make it clear which ones are now obsolete

[Cidolfas]

I added comments and Obsolete attributes to the unsupported C# backends to make it clear those are still around only as reference and are no longer maintained

I also updated the README to point to the valid examples, and fixed a compile issue in the ExampleSDL3OpenGL3 project so that anybody pulling the repository down for the first time should be able to jump right in