docs: add Antora documentation structure

[jimisola]

Description

The Maven plugin (reqstool-java-maven-plugin) uses Antora for documentation. The Gradle plugin should follow the same pattern for consistency across the project.

Current state

The Gradle plugin only has a README.adoc at the root level.

Expected state

Create documentation structure matching the Maven plugin:

docs/
├── antora-playbook.yml
├── antora.yml
└── modules/
    └── ROOT/
        ├── nav.adoc
        └── pages/
            ├── index.adoc
            ├── description.adoc
            ├── configuration.adoc
            ├── usage.adoc
            └── license.adoc

Implementation notes

• Migrate content from README.adoc to appropriate documentation pages
• Update the documentation to reference Gradle-specific details (vs Maven-specific in the maven plugin)
• Set up Antora playbook for publishing
• Consider adding to the main reqstool documentation site

References

• Maven plugin documentation: https://github.com/reqstool/reqstool-java-maven-plugin/tree/main/docs