Document Plugin Best Practices

[jdaugherty]

Issue description

The Grails documentation does a great job on how to create a plugin, but after the upgrade to Grails 7 we have learned several best practices. These practices should be added to the documentation.

Some topics that need included:

1. Build: Using a multiproject build to separate example/test code from plugin code
2. Continuous Builds: Adopting CI via GitHub actions (https://github.com/apache/grails-github-actions)
3. Publishing: Publishing configuration (https://github.com/apache/grails-gradle-publish)
4. Init: Publishing a github template for getting started
5. Documentation: How to publish documentation
6. Development: Easy environment configuration by adopting sdkman
7. Licensing: Ensuring licensing is clear and recommending the ASF license by default
8. Build: Gradle multiproject design using composition instead of if/else in the root of the project
9. Discoverability: Adding the plugin to our index of plugins (https://github.com/apache/grails-plugins-metadata)
10. Advanced Design: when to include the Grails Plugin Gradle Plugin and went to not.
11. Testing: testing approaches and how to ensure the project is maintained longer term.