add document for GenAI

[LittleLittleCloud]

We are excited to review your PR.

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• [ ] Add desgin document for #7169

[luisquintanilla]

@NiklasGustafsson @stephentoub @SteveSandersonMS can you please take a look and provide feedback.

This is the design document for the GenAI library which consolidates the models ported to TorchSharp (Phi, Llama, StableDiffusion) into a single library for easier consumption.