Add Duo Workflow documentation in duo experiments docs

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/468508

We're making experiment features toggle a requirement for Duo Workflows.

This MR is resolving the second point in issue DoD.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

• Visit https://main-ee-168633.docs.gitlab-review.app/ee/user/gitlab_duo/experiments.html#gitlab-duo-workflow
• You should see GitLab Duo Workflow section