Improve contextual sidebar styling: gray background & border

What does this MR do and why?

This is a follow-up to !87817 (merged). In the former MR we changed the background color of the contextual navigation.
But as we have no gray nuance defined in the Pajamas Design System between $gray-10 and $gray-50 this MR will opt to use a new gray which should be exclusviely used for the contextual navigation.

Comparison from former MR:

Before ($gray-50)	Current [MR 87817] ($gray-10, $gray-50 border)	Proposed exclusive gray (#f5f5f5, #e9e9e9 border)

Screenshots or screen recordings

Before	After
Light mode	Light mode
Dark mode	Dark mode

How to set up and validate locally

Check out the branch and go to an issue/MR.

Related to #361641 (closed)