Document Plugin Best Practices

[jdaugherty]

Issue description

Throughout the upgrade of Grails 7, we have discovered several best practices for plugins. The purpose of this ticket is to document those practices and add a section to the Grails Guide for guidance. The intent is to limit the need to update these plugins in the future via this guidance.

Some of these practices include:

• using provided scopes such as 'compileOnly' to not tie to a specific coordinate name
• splitting the plugin logic from it's examples. i.e. the plugin gradle project should only include plugin code
• ensuring github actions are setup to build your plugin
• using grailsPublish to ensure your sources, javadoc, groovydoc, etc are published correctly

[jamesfredley]

https://github.com/apache/grails-core/discussions/15044 - includes this list for now, it should like make it into https://grails.apache.org/docs/snapshot/guide/plugins.html