Add shell for learn gitlab redesign into get started

What does this MR do and why?

Add shell for learn gitlab redesign into get started

• redesign of the learn_gitlab path. Using a new controller/route due to ease of rollout, changing of url and MR reviews
• basic bits of controller/route and guard with feature flag
• see https://gitlab.com/gitlab-org/gitlab/-/issues/537653
  • specifically Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/538217

Screenshots or screen recordings

Blank slate for first iteration

How to set up and validate locally

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. In rails console enable the feature flag

   Feature.enable(:learn_gitlab_redesign)

3. Create a new group and project.
4. In rails console, onboard that group that was just created

   ::Onboarding::Progress.onboard(Group.last)

5. Go to the get started path for the group/project: For instance, http://gdk.test:3000/logan225-group/logan225-project/-/get_started

MR acceptance checklist

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

Related to #537653