Add "Contributing to GRIT" to GitLab docs (html)

This issue tracks adding comprehensive contributor guidelines to the GRIT documentation, including:

• Developer Certificate of Origin and licensing information
• Code of Conduct reference
• Contribution guidelines and best practices
• Testing and linting requirements

This documentation will standardize the contribution process and help onboard new community contributors.

Reference: gitlab-org/gitlab-runner!5263 (comment 2289854802)