[Docs] Package Documentation

[tjwald]

We need detailed docs for both usage and Generated API documentation.

This means:

• Core abstractions walkthrough
• NLP package usage
• Performance tweaking guide
• ONNX Runtime usage - especially how users need to add the specific backend in the user's csproj - gpu, cpu, ...
• Review all public APIs -
  • Change public to internal
  • Add docs to all remaining APIs
  • Generate api reference automatically