Keep visual edge with icons and chevron

What does this MR do and why?

Tweaks the margin of the section separators in the super sidebar to keep a visual edge with the icons of the nav items. Part of #402261 (closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

• Feature.enable(:super_sidebar_nav)
• Toggle on "New navigation" (in the user dropdown)
• Sidebar on the left :)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #402261 (closed)