added plugin documents

[Ajay9704]

added plugin best practices documentation

i worked on adding comprehensive documentation for plugin best practices as mentioned in issue #15326. honestly it took me quite a while to understand all the different aspects of plugin development in grails 7 but i think i got most of it covered.

what i did:

• wrote up how to set up multiproject builds with gradle to keep plugin code separate from example apps and tests
• included github actions setup for ci/cd (copied from the apache repos to make sure its correct)
• documented the publishing process using grails-gradle-publish plugin
• added info about creating github templates for new plugins
• explained how to publish documentation with github pages
• put in some notes about using sdkman for environment setup (makes life easier)
• wrote about licensing recommendations (apache license seems to be the way to go)
• included guidance on gradle composition vs conditional logic (learned this was important)
• added how to get plugins listed in the grails plugin index
• documented when to use the grails plugin gradle plugin vs simpler approaches
• wrote about testing strategies for keeping plugins maintainable long term

the documentation is in the usual grails doc format with asciidoctor syntax. i tried to make it practical with real examples instead of just theory. hope i got everything right - please let me know if anything needs fixing.

files i touched:

• added new file: grails-doc/src/en/guide/plugins/pluginBestPractices.adoc
• updated toc.yml to include the new section
• updated plugins.adoc to link to the new content

[Ajay9704]

@jdaugherty can you trigger the workflows so i can see the build status

[Ajay9704]

@jdaugherty all checks are successful , please review my pr and let me know if changes are required from my end