Add right sidebar for new learn gitlab

What does this MR do and why?

Add right sidebar for new learn gitlab UI. Resolves #538220.

Design here: https://www.figma.com/design/fenlPk0ZuX9F79sFMzemwk/Onboarding---Learn-GitLab?node-id=84-6833&t=ocuEG7r4O2SzvnGc-1

References

Screenshots or screen recordings

Before	After

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. In the gitlab admin UI, edit the group to be on an ultimate trial plan(edit button from http://gdk.test:3000/admin/groups)

6. In rails console(bundle exec rails c), change the trial, trial start and end date for the group

   Group.last.gitlab_subscription.update_columns(trial_starts_on: Date.today, trial_ends_on: 30.days.from_now, trial: true)

7. 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

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