Docs for GitLab Duo CLI

This issue is to create draft documentation for the GitLab Duo CLI.

• Epic is here: &19806

• Put new doc in: gitlab/doc/user/gitlab_duo_cli/

• In nav, put above GitLab CLI under Extend.

• Add Experiment as tier badge, add history note with link to epic for now.

Usually the developers draft the content, but in this case you can create a draft and ping the developers for review.

Other items to consider (Uma's notes):

• Do we need to update the ai_features entry in https://docs.gitlab.com/user/profile/personal_access_tokens/#personal-access-token-scopes because the CLI requires api scope and not ai_features only (see my comment here)