Document the process to be a maintainer / maintainer trainee / reviewer in AI Gateway

Problem

Reference https://about.gitlab.com/handbook/engineering/workflow/code-review/#how-to-become-a-project-maintainer

Proposal

• Document the process to be a maintainer / maintainer trainee / reviewer in AI Gateway
• Add Danger Bot for enabling reviewer roulette
• Document AI Gateway maintainers and reviewers (gitlab-com/www-gitlab-com!131876 - merged)