Add `Learn Gitlab` Menu to project sidebar refactor

What does this MR do?

This MRs moves the Learn GitLab menu from the HAML file to the new project sidebar refactor.

Screenshots (strongly suggested)

Before the MR	After the MR

There is a small difference in the HTML markup, but it's because there is a bug in the former code. When the sidebar is collapsed and we hover the element, the menu is not displayed, like we do for merge requests, for example:

Nevertheless, in this MR we fix that and the menu is displayed:

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because is a tooling change.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Related to #327444 (closed)