Add .NET AI templates to docs

[jmatthiesen]

We're close to releasing an initial preview of New Project (dotnet new) templates for AI development. Along with their release, we'll want to update the AI docs site to include:

• A Quickstart using the templates
• .NET AI Template overview page with instructions for environment setup (a manual process for now)

[jmatthiesen]

Alex published the first version of this doc and is now working on adding in Azure AI Search instructions. We're planning to handle that as a second document.

[JeremyLikness]

@alexwolfmsft what's the status of this?

[gewarren]

Ping @alexwolfmsft