Keep the header section always visible
Currently, the sidebar header disappears while scrolling down the sidebar. Having a fixed point of reference in the sidebar could move towards empowering the design system identity.
Eventually the sidebar and fixed point of reference could include the Pajamas logo and text instead of the GitLab logo and text. See discussion in !216 (comment 134508666).
A small improvement that could take place before #55 would be to change the header to a fixed position.