Document validation support for minimal APIs

In .NET 10 Preivew 3, we're introducing support for minimal API validation support.

I've created a sample repo and some notes on the implementation to serve as source material here: https://github.com/captainsafia/minapi-validation-support.

One thing to note is that this feature has a user-facing component for developers who are building minimal APIs and a framework component for developers who are building frameworks or their own validation libraries.

I think our first priority is developers building APIs since that's where the biggest product gap it exists.