docs(devkit): document broker log location

[superhx]

Summary

• Document that just logs only shows container stdout/stderr
• Add examples for inspecting detailed broker logs under /tmp/kafka-logs with just exec
• Use full Gradle build as the default devkit build command

[Gezi-lzq]

Reviewed. The README clarification about broker log locations matches devkit behavior, and the justfile build command simplification is consistent with standard local build usage. No blocking issues from my side.