Add guidelines for how to structure Java code

Describe the recommended package structure for organizing Java code.

Consider:
- package-by-feature
- Clean/Hexagonal Architecture

Add a detailed package structure for a sample multi-module Java application.

No target files or tests are named. Start by reviewing the repository's existing documentation structure, then define the sample multi-module Java application's package layout and explain package-by-feature alongside Clean/Hexagonal Architecture. Done means the guidelines contain a detailed, understandable structure covering the requested approaches.

Written by the indexing model from the issue text.