Document validation support for minimal APIs

[captainsafia]

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.

[guardrex]

@captainsafia ... I'll move this one as well. Dan would like to track each preview's work items on one issue for visibility to make sure nothing falls through the cracks.

[captainsafia]

@guardrex Sure -- I surfaced this here since we'll want to track documenting this permanently as part of the minimal API docs before we GA. This is in addition to the point-in-time stuff for Preview 3 what's new.

[Rick-Anderson]

@wadepickett use Ctrl+Alt+G on the appropriate doc to add meta-data to this issue.
Update the bottom of this comment with 2 check boxes, this issue and the WN.3 issue.