Add kairo-admin dashboard and kairo-kdocs integration

[adrw]

Summary

• Adds kairo-admin and kairo-kdocs dependencies to demonstrate how easy it is to integrate the new admin dashboard and Dokka-powered API docs into a Kairo application
• Creates a kairo-sample-dokka convention plugin (mirrors the kairo-service-dokka plugin from kairo#1149) to generate and package KDocs into the classpath
• Wires up KdocsFeature and AdminDashboardFeature in Main.kt — dashboard available at /_admin, docs at /_kdocs

Test plan

• Verify Gradle sync succeeds once kairo-admin and kairo-kdocs artifacts are published
• Run the app locally and confirm the admin dashboard loads at /_admin
• Run ./gradlew dokkaGenerate :packageKdocs and confirm KDocs are served at /_kdocs

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