Adding a .NET + AI Ecosystem doc

[jmatthiesen]

Summary

This is a new document summarizing the .NET + AI ecosystem, to which we'll link from the .NET + AI landing page and other documentation.

---

Internal previews

📄 File	🔗 Preview link
docs/ai/dotnet-ai-ecosystem.md	Overview of the .NET + AI ecosystem

[gewarren]

I left some suggestions to consider. For links to other learn.microsoft.com resources, we're required to omit the first part of the URL so the links work in air-gapped environments.

[codemillmatt]

Couple of changes - next steps and update date

[gewarren]

My only other suggestion is to name the last column in both tables "Link to docs" and then use the title of the doc as the link text in each row instead of "docs". But up to you.