doc(java-generator): add configuration examples for plugin options

[taole33]

issue

#5285

Description

This PR improves the documentation for the Java Generator by adding configuration example snippets in the Usage section.

Currently, the documentation lists available options but lacks practical examples of how to configure them in pom.xml or build.gradle.
This update provides concrete examples for Maven, Gradle, and CLI usage, along with the expected generated Java code for the following common options:

• Uppercase Enums (enumUppercase)
• Package Overrides (packageOverrides)
• Always Preserve Unknown Fields (alwaysPreserveUnknown)
• Filter Source Files (filesSuffixes)
• Existing Java Types (existingJavaTypes)

This addresses the missing link between configuration and output as pointed out in #5285.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change
• Chore (non-breaking change which doesn't affect codebase; test, version modification, documentation, etc.)

Checklist

• Code contributed by me aligns with current project license: Apache 2.0
• I Added CHANGELOG entry regarding this change
• I have implemented unit tests to cover my changes
• I have added/updated the javadocs and other documentation accordingly
• No new bugs, code smells, etc. in SonarCloud report
• I tested my code in Kubernetes
• I tested my code in OpenShift