Update AI feature Development Playbook
Based on gitlab-org/ai-powered/discussions#18 (comment 2067647452), lets move this to the public docs. See https://gitlab.com/gitlab-com/content-sites/internal-handbook/-/merge_requests/5373+s
Based on gitlab-org/ai-powered/discussions#18 (comment 2067647452), lets move this to the public docs. See https://gitlab.com/gitlab-com/content-sites/internal-handbook/-/merge_requests/5373+s