Add contributing section for GRIT docs
What does this MR do?
Adds a "Contributing to GRIT" section to the documentation, covering the Developer Certificate of Origin, Code of Conduct, contribution guidelines, and testing/linting requirements.
Why was this MR needed?
To provide clear guidelines for community contributors and standardize the contribution process according to GitLab practices.
What's the best way to test this MR?
Review the rendered documentation to ensure proper formatting and verify that all links to external resources are working correctly.
What are the relevant issue numbers?
Edited by Davis Bickford