Add Maven Central publishing design document

Documents the requirements and implementation plan for publishing
PerlOnJava to Maven Central, including:
- POM metadata requirements
- GPG signing setup
- Central Portal account setup
- CI/CD release workflow for all platforms

Generated with [Devin](https://cli.devin.ai/docs)

Co-Authored-By: Devin <noreply@cognition.ai>

Author: fglock