Add Storybook documentation for Vue shared components to navigation

Add navigation item to global navigation

Storybook is still recommended way to document Vue shared components, but not many team members know that exist. We discussed making it easier to discover, by adding it to the navigation under Frontend Guidelines.

• Link to the merge request that introduced the new page: n/a (the page has already existed but wasn't included in the navigation)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://psjakubowska-main-patch-12618.docs.gitlab-review.app/ee/development/fe_guide/storybook.html