Review and Update Contribution Guide at Grails Documentation

[puneetbehl]

Issue description

Description:

The current contribution guide located at https://docs.grails.org/latest/guide/contributing.html might be outdated. To ensure that contributors have the most accurate and helpful information, we need to review and update this document.

Tasks:

• Review the current contribution guide for outdated information.
• Identify sections that need updates or additions.
• Update the guide to reflect the latest contribution practices and repository management.
• Ensure all links and references are current and valid.
• Add any new information that can help contributors, such as updated coding standards, new communication channels, or changes in the contribution process.

[jdaugherty]

#13853

[jdaugherty]

Updated the github grails/grails-core CONTRIBUTING.md and referenced it via external sources. I left the github how-to on the official website in case people aren't familiar.