Add Kotlin native nullability documentation

[taefi]

Summary

• Adds comprehensive documentation for Kotlin's native nullability support in Hilla 24.8+
• Updates FAQ with reference to the new Kotlin nullability documentation

Fixes #4421

Changes

• articles/hilla/lit/reference/type-nullability.adoc: New section documenting Kotlin's native type system support, including:
  • Language-based nullability detection using Kotlin's ? operator
  • Maven configuration requirements (-java-parameters compiler flag)
  • TypeScript type generation examples
  • Nullability priority rules for mixed Kotlin/Java codebases
• articles/hilla/lit/start/faq.adoc: Added reference link to the new Kotlin nullability documentation

Test plan

• Review documentation for technical accuracy
• Verify Maven configuration examples are correct
• Check that code examples compile and generate expected TypeScript types
• Ensure cross-references between FAQ and type-nullability pages work correctly
• Validate that the documentation builds without errors

🤖 Generated with Claude Code

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[github-actions]

AI Language Review

• FAQ.adoc: The update adds a reference to "Kotlin's Native Nullability Support" in the question "Does Kotlin, Scala, or Groovy Work with Hilla?" Ensure that the link placeholder is correctly resolved to avoid rendering issues. Ensure consistency between different parts of the document when adding new references.

• Type Nullability.adoc: The added section on "Kotlin Native Nullability Support" includes detailed information on handling nullability in Kotlin. Check that the added explanations and examples are clear, and that the Markdown syntax is correctly rendering notes and code blocks. Also, make sure that the references to Maven and Gradle configurations are correctly formatted for inclusion in a technical document. Ensure that any placeholders used in links or text are correctly filled to prevent rendering issues.