docs: add Gradle Kotlin quick start

[adity982]

Summary

• add the Gradle Kotlin DSL test dependency requested in Docs: Gradle (Kotlin DSL) install snippet and a Kotlin usage example #15
• add a copy-ready Kotlin @McpServerTest quick start mirroring the Java example
• keep the example focused on the existing JUnit 5 extension and fluent assertions

Validation

• mvn -B test (45 tests passed)
• git diff --check

Closes #15

[adity982]

Closing this later draft as a duplicate of #16, which was published first for issue #15. I will keep validation and follow-up on #16 so maintainers have one review surface.