Document why NativeValidation is needed for Native AOT

[kzu]

Summary

• Explain the Native AOT and trimming limits of System.ComponentModel.DataAnnotations.Validator, including the RequiresUnreferencedCode annotations and the scope of the options and ASP.NET generators.
• Add Why, What, and How sections to the main readme, with usage samples and the source generator's open-generics path.
• Record the primary-source notes in docs/dataannotations-aot.md.

Test plan

• Read the Why, What, and How sections in the readme and confirm the samples match NativeValidator.
• Confirm the NuGet readme include still pulls the #content region.

[kzu]

🧪 Details on Ubuntu 24.04.5 LTS

from retest v1.1.0 on .NET 10.0.12 with 💜 by @devlooped