fix: 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

[github-actions]

AI Language Review

In the FAQ file, there is a newly added reference link in the section "Does Kotlin, Scala, or Groovy Work with Hilla?" which points to an article titled "Kotlin's Native Nullability Support," but the syntax for cross-references within AsciiDoc (like <<{articles}/...>>) is not self-explanatory without proper setup. Ensure this placeholder is correctly resolved or clarify how it relates to the document structure.

In the Type Nullability section, the content about Kotlin nullability has been significantly expanded. While no specific issues were noted in the changes, ensure that all references to external dependencies, like hilla-parser-jvm-plugin-nonnull-kotlin, and plugins are correct and accessible. Additionally, verify the cohesiveness of the Maven and Gradle setups as they relate to these updates on nullability handling.