Revisit sidebar peek behavior

In the new Project Studio UI, we don't have the old design's "peek" behavior in the lefthand sidebar.

We got some user feedback asking if it we could bring it back:

• https://gitlab.com/gitlab-org/gitlab/-/issues/564999#note_2807559438

Let's wait for public beta feedback if more users ask for it and decide if we again want to support both hover-the-toggle-button peek and hover-the-window-edge peek, or only one of the two.

See first linked feedback for some more considerations.