docs: add Kotlin quick start

[adity982]

Summary

• add the Gradle Kotlin DSL dependency for release 0.5.1
• mirror the Java @McpServerTest quick start in Kotlin

Validation

• mvn -B test with Temurin JDK 17 and Maven 3.9.16 (45 tests passed)
• git diff --check

Closes #15

[senor14]

Clean first contribution — correct Kotlin DSL syntax, the annotation array form, and you even switched the jar path to Gradle's build/libs/. Thanks for validating with the full test suite too.